The UK has already tried to reform the EU, and failed because too many of the other member countries like it the way it is for their own interests and as more less wealthy nations join up it will then become impossible for the UK to influence changes from within the EU, so leaving will make no difference to this aspect.
Right now European Union is a dictatorship run by Germany, with the other nations having no voice over the policies and important decisions. This is the reason why many other member nations (in addition to the UK, the Dutch and the Spanish are also seriously reconsidering their membership) are thinking about exiting the European Union.