There are some in Britian, maybe lots, who want to leave the EU.
There's a good piece on what might happen here, but it's all very speculative. What would happen? No-one knows.
If we stayed in the common market, we'd just loose influence.
If we left that too, we'd be alongside the other non-EU European powerhouses like Norway (although it is "required to adopt much EU legislation due to its participation in the European Economic Area (EEA), through the European Free Trade Association (EFTA)"), Switzerland, Iceland etc.
So, we stay. Changes we want to see, we make from the heart.
On another small point, if David Cameron is the champion of the poor, why is he desperate to get out of European worker-protection legislation?