Leavers believe that the People are sovereign, Remainers believe that Parliament is sovereign
The UK is a representative, parliamentary democracy, and as such it is Parliament that is sovereign.
Remainers are A1 OK with individual states subsuming themselves into a European superstate; Leavers want to be able to vote their law makers in and vote them out.
The individual states are not subsuming themselves into a superstate and each country has a veto. As a Europhile, I support the idea of 28 States acting together for the common good, but I also believe that the EU could be improved.
Most of the laws passed in the UK come from the UK Parliament. EU laws are passed by the elected representatives, MEPs, which include British MEPs. It is the British second chamber, the House of Lords, which is not elected (see May's latest list of enobled cronies) and the UK does not have an elected Head of State.