There is always a trade-off between sovereignty and national wealth. Any trade deal affects sovereignty because it requires accepting compromises. The modern, globalised world made possible by the likes of the EU single market and customs union has empowered UK-based manufacturers (most of whom are foreign-owned anyway) to develop supply chains stretching across many countries. If we insist on "sovereignty" and "independence" then these manufacturers will move abroad, to countries where their supply chain models still work, rather than trying to vault the barriers we put in their way, unless we absolutely slash corporation tax which will in turn entail drastic cuts in public services. Many other international bodies such as the United Nations, NATO, World Trade Organisation and the International Criminal Court affect our ability to "make our own laws" as well. We have to accept that supra-national laws are here to stay and are part of 21st Century life: the only question is whether we have a say in making them. We had and still have a very big say in making these laws as part of the EU: before the referendum we were on the winning side in about 95% of EU votes. By pursuing the policy of leaving the EU, the UK is ironically and somewhat counter-intuitively losing sovereignty, because we will almost certainly be subject to laws and standards over which we have no influence any more. We also run the serious risk of being increasingly ignored on the greater international stage, as was shown at the G7 meeting in April 2017. Any discussion of "sovereignty" needs to acknowledge that the UK's influence on world affairs was greatly amplified by our role as a leading member of the EU and that the aftermath of the Suez Crisis in late 1956 showed that the UK on its own was not a major global power even 60 years before the EU referendum and that even then it was clear to us that the future lay in political cooperation with other European countries, rather than trying to be a "great imperial power". In a massively globalised world, it's impossible to "take control" because the global manufacturing chains that create wealth will just bypass us in favour of territories that offer harmonised legal environments for them to operate in.
One of the topics often raised in the referendum debate was "sovereignty" - expressed in forms such as "I want to see laws made in this country!" or "taking our country back!" When debating the idea of sovereignty, we need to decide what it means: does it mean the ability to be in control of our own destiny (which we always were because of the democratic nature of the EU and the influence we had on those democratic decisions), does it mean the ability to exert our influence on global affairs (which is fast-fading as Brexit makes us look like an irrelevance on the world stage and drastically undermines the two main pillars of UK foreign policy, namely the ability to influence other European nations' decision making processes and as a result to have the ear of the US Government), does it mean the ability to act unilaterally (an impossibility in the modern world an an ability which we have been less and less able to exert with each international treaty and trade deal we have signed ever since the still-active Anglo-Portuguese treaty of 1373)?
Contrary to how it is viewed by many people, the EU is not an "evil empire" which actively seeks to harm us and force its rules upon us. It is an alliance of member states who have decided to pool their resources (and to an extent, their sovereignty) in order to speak with one common, powerful voice on a variety of subjects and to establish an internal market that is as free of hindrances and friction as possible. To do this, the EU needs its own laws but those laws need to be agreed by the member states of the EU (including the UK). The UK isn't marginalised or outvoted all the time in the EU: we often get our way or persuade others to come round to our way of thinking, helping to shape the final outcome to the way we want it. EU laws aren't forced on us because we agree to the vast majority of them and very often have a leading hand in writing the proposals that are being debated in the first place, such as the influence a UK public campaign had on the reform of the EU Common Fisheries Policy or how EU policy on scientific research and testing of new medicines is heavily influenced by the UK. By pooling our resources with those of other EU member states, and it might be said giving up some of our individual sovereignty (in terms of the ability to act independently and unilaterally), we gain far more than we lose. The failed UK attempt to take control of the strategic Suez Canal by military means, and subsequent international reaction from more powerful countries such as the US, in 1956 led to a reassessment of UK foreign policy objectives and arguably made UK politicians realise that the international fortunes of the country were best served by alliances with other European countries, rather than attempting to be an imperial power when Suez had made it obvious that other countries disapproved strongly of the UK continuing to promote "empire era" foreign policies. In fact, many historians argue that the humiliation of Suez is what caused the UK to first apply to join the-then European Economic Community. Ultimately, even in mid-1950s world politics, it was obvious that the UK alone did not have the clout to be a significant player on the world stage and the passing of time has not altered this. Our place in the modern world is as a leading player in one of the world's leading power blocs, namely the EU, with a powerful say in the direction of that organisation, rather than as a buccaneering solo entity, which modern global politics doesn't permit anyway.
The UK is subject to the rules of other supra-national bodies aside from the EU. Each of these bodies, such as the UN, NATO, the International Criminal Court and the hundreds of international treaties concluded or ratified by the UK, some of which relate to these bodies and others of which are bilateral agreements with other countries or international agreements such as on climate change or "intellectual property". The important thing to note is that they all impinge on sovereignty (if we define sovereignty as the ability to act unilaterally or decide upon our own destony as a country). Every one of these treaties restricts our ability to act independently. This is particularly the case with trade deals: every trade deal affects sovereignty because - as the term "deal" implies - every trade deal requires making compromises with the other negotiating party or parties. The more comprehensive the deal, the more compromises have to be made and so the more "sovereignty" is diminished. The EU single market is the most comprehensive trade deal known, because it seeks to eliminate not just tariffs but other barriers to trade such as regulatory differences and incompatible national standards. This all means more agrrements to be found, more compromises to make and more loss of "independence" and "sovereignty". However the effect on the overall economic health of the country, and in turn each of us as individuals, makes the "loss of sovereignty" worth it - as does the fact that the UK has full input into the creation of all EU single market rules as a full member of the EU (as opposed to countries such as Norway and Switzerland which must effectively accept these rules without being able to shape them).
It's also worth reading up on how incredibly complex global trade is in the modern world - Nissan's car plant at Sunderland has a supply chain stretching across 24 different countries, one single component of a modern Mini car crosses the English Channel three to four times between the component being manufactured and the finished car ending up with an owner, popular alcoholic drinks have production processes involving both sides of the Irish border before they are exported to EU customers and an iPhone might be designed by Apple in Cupertino but its components come from numerous different suppliers prior to being assembled into the finished device. All this results in better quality, more technically advanced products for us as consumers but behind the scenes these supply chains require the certainty of common regulations, just-in-time deliveries to keep stock levels low which in turn require the certainty of no holdups for customs formalities at international borders, and ultimately supra-national legilsation to guarantee no disruption and to ensure good matching of the regulations surrounding the manufacture of each component.
Modern trade isn't about "raw materials imported, finished product exported" or staple goods being shipped between individual countries, but more about "global value chains" which are controlled by multinational companies and regulated by what to the casual observer is an impenetrable web of public and private regulation. The task facing the UK, and other countries in the world today, is to find a way of harnessing these chains to create jobs and wealth in our neck of the woods: but since these value and supply chains are largely controlled by giant multinational companies, we have no choice but to seek agreement from other countries - for example, Nissan would obviously be concerned about import tariffs on both complete cars and individual components if we leave the EU and its single market, but they will be even more concerned about European automotive emissions and safety standards, business accounting standards (all currently harmonised via the single market but in danger of drifting apart if we leave the EU) and the ability to hire the best talent from all of Europe (especailly if freedom of movement is clamped down on). The multiple intermediate steps that the likes of Nissan, Toyota, Honda and other UK-based car manufacturers must go through between sourcing their components and driving the finished vehicle off the production line are what govern their decisions on where to make costly, long-term investments in factories and jobs, and they will choose these locations according to the regulatory environments they need to operate under. In such a highly connected world, where power lies arguably just as much with businesses as it does with government, what does "sovereignty" mean?
The UK Government White Paper on Leaving the EU noted that “Whilst parliament has remained sovereign throughout our membership of the EU, it has not always felt like that.” Equally, Theresa May's assertions that the UK will be "a global leader in free trade" after we leave the EU are not only wildly optmistic (see our article on "Going global?") but also utterly incompatible with the idea of being sovereign, since no other country is going to accept our terms for trade without even trying to get their own wishes and agenda accommodated. Modern, instant communications technology has made fragmented and distributed manufacturing and assembly techniques possible. To be a wealthy, successful country we need to persuade the companies in charge of these processes, both the individual components but especially the high-value end products, to locate the processes in the UK. "The more trade we desire, the more these rules proliferate, because the fragmented and infinitely complex nature of modern production, distribution and consumption requires ever-more esoteric and finely grained forms of governance." And the greater the effect is on "sovereignty".
Think of it this way. We, as individuals, go to work either for an employer or running our own affairs if we are self-employed. This affects our "sovereignty" because it takes a chunk of our time each day and prevents us from doing what we might want to do with that time if we didn't have to go to work. But the vast majority of us accept this trade-off because even though we lose the ability to decide for ourselves what we want to do with the hours we spend working, we get paid for the time we devote to our employer and this enhances our quality of life in the time that is still ours to do with what we please as well as cover our costs of living ("You can't eat sovereignty!"). So it is with our country as a whole: we give up some measure of independence (or "sovereignty") in order to increase our national wealth by means of trade deals and other international agreements. North Korea is a fine example of a sovereign state: it has no friends and no international relations to speak of. Its only link with the outside world comes from frequent demonstrations of military strength, yet meantime its population starves. You just can't eat sovereignty, after all.
P.S. If you're struggling to understand all of this, don't worry overly - just ask yourself why such a complex question was ever put to the general public in the first place?