The UK is unlikely to get a good free trade deal with the US, China or Japan on favourable terms, though. Being part of the EU offers much better prospects of a good deal, simply because the EU is a much bigger player (the biggest, in fact).

It may not be fair, but on the world stage, power matters. Inside the EU, we try (but don't always succeed) to be fair, but outside of it, power matters. A relatively small country like the UK doesn't have much leverage to get a good deal from other major powers, whereas the EU does.