Trade and cooperation between two countries, two people, two entities is reducible to or can be likened to division of labour. Like in, a full time carpenter still needs food, so he has to go to a market supplied by a full time farmer who goes to a market supplied by a full time carpenter for his furniture needs. Using such an example, it is clear that free trade enhances prosperity and that everyone gains when a market deal is concluded. No losers.

Easing trade and migration will help everybody, provided that all nations stick to thesame basic economic system. We are all humans. If there is something the EU is not doing right, it is better to suggest a way to do it better, then cooperate with them. Unrestricted Trade and cooperation enhances prosperity. That was Adam Smith's gospel and he was a Briton.
Personally I would say Britain should insist on an EU parliament where representation is based on GDP size. Turkey should be admitted. If you don't like Islam, then you shouldn't like Christianity as well. Abandon your own Christianity and advise the Turks to abandon Islam for a sounder concept of God as a gentleman. Britain rode to prominence on trade with Europe. It was British clothiers looking for ways to supply Europe woollen cloth at a competitive price that was a major factor that led to the industrial revolution. If not for the European cloth market that spurred the industrial revolution, the British empire might have ended up like the Spanish empire before it.
The EU is a trading bloc and not a country.
Europe has no official language or national identity and what is happening in Europe is what the whole world should copy.

What has Britain got to gain? Access to markets, then a shared defense budget instead of the huge British defense budget leading to more funds freed for human development in Britain. The benefits of cooperation are limitless and unquantifiable.

What has Britain got to lose? Some say control over immigration policy but the EU is not poor and if Germany is not complaining, why should Britain? Also, the British themselves have the highest number of nationals per capita of all European countries, who are expatriates all over the world.