An interesting summary of referendums in the UK and their legal status, especially pre legislative and post legislative referendums.

One is left with the feeling that a) the result would be ignored.
b) the vote would overwhelmingly be to remain. The fact that the people voted to leave is the only explanation for camoron scuttling off into the wilderness. This is because a leave vote would have to be ratified by Parliament, and there is no way this could be guaranteed.
The entire exercise was a sham, but the government bluff was called when the majority voted for exit.

https://en.wikipedia.org/wiki/Referendums_in_the_United_Kingdom

(It is dangerous to use wikipedia as a source, but in this case I believe it is a clear summary)