Because referenda on the Lisbon Treaty are not referenda on EU membership despite everyone and their mother wanting to interpret them that way, whereas the 25% combined vote total by the UKIP and BNP is an explicit vote against EU membership since that's what those parties campaigned on. And the other 25% of Tory vote is also against the EU though as TBG reminds us, if push came to shove the Tory party probably wouldn't advocate withdrawal outright.

Granted, turnout was low, but so it was in every other EU member state. And when people tout the 70% majority for Lisbon in the Spanish referendum, they forget to mention only 40-45% voted, too...

The brainless should not be in banking. — Willem Buiter