EU thought experiment

Imagine that the European Union does not exist, and never has done (a nightmare scenario for some, a beautiful dream for others!) Now imagine that another country (let's say Switzerland, for example) offered to pay the United Kingdom £70 billion per year on condition that we copied all of Switzerland's laws and implemented them into UK law - but only in certain fields (say, human rights, health and safety, immigration, employment rights and VAT) and that, following a referendum, the UK agreed to enter into such an arrangement, on the understanding that our parliament could vote to break off this arrangement at any time.
Now, it is perfectly possible that a majority of people in the country might not like one - or even all - of the laws made for us by Switzerland but still choose to retain the arrangement in order to receive the £70 billion annual payoff. And it is equally possible that a majority could decide that the Swiss-made laws were too onerous and no longer worth the money, in which case they would elect a government to take us out of the arrangement with Switzerland.
The point of this analogy is not to argue that the UK benefits financially from the EU (although I am pretty confident that it does, through the single market and its associated advantages) but to show that at no point in the scenario described above, whether the UK public like or dislike the legislation imposed on them by Switzerland, whether choosing to retain or abandon the arrangement with Switzerland, does the Westminster parliament cease to be the sovereign decision-making authority for the UK and at no point does democracy cease to operate or the will of the UK electorate cease to be implemented.
What this shows is that by 'contracting out' some legislative powers to the European Union, the ultimate sovereignty of the UK parliament in Westminster is in no way negated or diminished.