"We could, I suppose, all vote for UKIP, but its unlikely that they will get enough MPs to form a Government. "
If we all did they could not fail to do so, Mikey !! In fact, they'd get 650 seats!
I think even Mr Cameron has decreed that if the Brexiters prevail then that will of the people will be transformed into the UK leaving. (Mind you, I think what he said was "whatever the will of the people it will be transformed etc. etc.). This was when he thought there was not a cat in hell's chance of a Leave vote.
A constitutional crisis would certainly ensue if the vote was ignored. In short, there would be ructions. Parliament's job, in the event of a Leave vote, would be to disentangle ourselves from the unholy mess that is the EU asap. That's what they are paid for and that's what they must do. Either that or step aside and let somebody else do it.