Suppose the EU collapses after we leave.

Why should it? If we are going to base the argument on a "What If" premise then I think there are far worse what if scenarios than that to scare us with. Besides I thought those campaigning for us to remain believed that the EU is all things to all men, perfectly wonderful and we would be fools to leave.

We could find a Europe in economic turmoil comprising disparate states with huge inequalities between rich and poor countries with their no longer enjoying mutual interests or a desire to prevent the weakest from becoming basket cases.

"We could find!!!! you have got to be joking. I think we are there already ask the Greeks. Only the interests of two countries have ever been considered by the EU, those of France and those of Germany.

Wouldn't do much for our trade either.

Surprisingly enough I do not believe that that is true any longer.

We would quickly be back to pre-WW1 and pre-WW2 status in many regards.

I do not believe that there is any way that the British Empire would be recreated, although we would be free to trade with whoever we wished, we did so before.

There could easily be a threat to democracy in a number of the poorest nations.

Always has been and guess what? Nothing we could do to stop it due to the number of anti-war protesters who vehemently object to any sort of foreign intervention.

The Common Market was set up just twelve years after WW2 and the explicit aim of its early-fifties forerunner was to integrate nations in order to prevent future wars.

Correct but add the final bit ....to prevent future wars between France and Germany

Despite the ludicrous award of a Nobel Peace Prize the EU has not ensured peace in Europe it was totally powerless to influence what happened in Yugoslavia. The EU has not faced down any threat, it is unable to. The only major threat to western democracy since 1945 has been posed by Russia (Ask some of our newer eastern European EU members) and the only thing that has held Russia in check has been NATO and the existence of nuclear deterrence.

On that subject, what big chunk of sovereignty has been surrendered to the United States of America? And how did they involve us in a 45-year "Cold War"? For there to have been an insane arms race, then there had to be at least two participants and a reason for them to be racing.