Eu referendum whats your vote

Answers in simple English please.
If Britain wants out,(and I respect their right to have voted out),and lots on here agree they are a net contributor,why oh why arent they saying to the EU,were out.
Like what is actually stopping them from walking?
Surely there must be some element of truth in moneys being owed,or whatever phrase you want to put on it.Or some legal aspect?
If theyre so sure theyre right why not do a Harvey Smith on it?