I'm not GP, but the UK has negotiated four opt-outs (you could also include Cameron's pre-referendum requests). This is more than any other EU country.

The economic and monetary union (the euro), Schengen, the Charter of Fundamental Rights and "the area of freedom, security and justice".

The rebate is (was) the biggie.

Britain was allowed to not follow many EU laws, for example they were allowed to keep the pound and not join the Euro as the treaties mandate.