UKIP could have been stopped by any governing party worth its salt by showing them up as the petty xenophobic racists that they are.

You forget how rude the Tories were about them. They did accuse them of racism and called them "swivel eyed loons."
The fact remains that outside of Westminster most people did not like being in the EU.
UKIP offered a referendum and that would have won them a large vote in the general election. They had already won the EU election.

That is why all the parties, not just the Tories, also promised a referendum.