Jacob Levy has a very good post on this topic, here is one bit from it:
There’s a level of popular belief that the EU enforces illiberal and market-unfriendly policies on Britain. On the fiscal side, here’s a comparison of British public spending as a share of GDP just before entry into the EU, and just before the Brexit vote:
Even when you add in the <1% of GDP that is paid to Brussels, this is just not a picture of a system that has forced Britain to become a big-spending social democracy. (Neither, of course, is it a picture of a system that has forced Britain into neoliberal austerity, a charge one hears from the left.)
Here is another:
According to the 2015 Economic Freedom of the World report’s overall measure for regulatory burden, Czech Republic, Denmark, Estonia, Lithuania, Sweden, Ireland, and Romania are all less regulated than the UK. The most recent Heritage index of “business freedom” ranks Denmark, Finland, Germany, and Sweden ahead of the UK; for labor freedom, Denmark, Austria, and Ireland. In all these cases, these relatively-liberal EU countries compare favorably with other developed countries in or out of the EU.
Do read the whole thing, here are comments from Ilya Somin.