The EU project will collapse if the UK leaves!
With the cost of all the new 'visitors' and the PIGS on the verge of doom once more, the money magicians cannot keep pulling money out of the hat to sustain the weaker parts, the devalued €uro will hinder things further.
A northern alliance lead by France and Germany may arise from the ashes but in time that could cause problems and aggression, the main reason for the zone originally was no more war, a Germany on Steroids and a France with a BMW backbone might cause grief in the future.
The UK like Iceland will be fine, it had an empire, I'm sure it hasn't forgotten to make trade deals...
And the consolation prize, we could get to beat up the Germans again