a little of both.
basically, by being in it, they are subject to laws of the 'union' in terms of trade, tariffs, immigration, money, etc.
As one of the few power nations in the EU, they feel they are giving too much and getting to little in return, so they feel they can do better on their own outside. They will make their OWN laws that suit themselves, not the union which has been plagued by terrorism and financial problems.
I agree and think it was a smart move. There's a lot of turmoil right now in europe not just in greece or a few restricted places... it's basically everywhere. (remember the Spain top gear special?)