But - I think you are referring to the state of the UK when it joined the EEC in 1973. The UK was in fact in a big mess at that point - which promptly got a lot bigger. This led to IMF intervention in 1976 [1]. This was not a big win for the EEC at that point in my book. The UK was in a big hole because it had spent all its money on unwinding its empire and failed to either properly retool its industries or pay down its second world war debt. The EEC and Europe in general did not help or bail out the UK - in fact they continued to expect payments from the UK in the midst of all this, a pattern that I fear Italy and Spain are about to encounter in the way that sentances encounter full stops.
What turned this round - the discovery of North Sea Oil. This enabled Thatcher to restructure the UK economy, recapitalize the City of London and hey presto off the UK went to the races. The big buggeration was the collapse of the Soviet Union which meant a restructuring of Europe in order to prevent the East from falling into anarchy and exposing the German state to abject collapse. Thus we all got swept up into the EU.
Anyway, if you want to argue about "prospered inside the EU sad much as the UK has" I ask you to look at the economic performance of Germany.