Spain and the UK are both large (by European standards) economies. Both of them started the crisis with current account deficits and suffered a real estate bubble prior to the crisis. There are, of course, differences: the UK financial sector is larger than that of Spain; the real estate bubble was larger in Spain,... But the difference that tends to be highlighted more by economic commentary is the Euro membership. Spain could not depreciate its currency as the UK did, Spain cannot resolve its government debt problems by relying on its central bank, and Spain could not lower the interest rate to zero as the UK did. Are these factors visible in the evolution of GDP from the beginning of the crisis to today?
This is clearly not a scientific proof that Euro membership has not made a large difference (I am not controlling for all the other factors). But at a minimum, it raises questions about the statement that membership in the Euro area is a key factor to understand the performance of Euro members during the crisis.