How can the London Evening Standard justify excluding UKIP from their Mayoral debate?

The London Evening Standard are having a debate. The choice to be put to Londoners is between Boris Johnson, Ken Livingstone, Brian Paddick and Green candidate Jenny Jones.

Luckily for those in London that isn't the actual choice facing them - now that would be a scary thought. However, it is the choice that the newspaper is presenting to voters, despite the fact that they recently ran an article pointing out that UKIP and the Greens are snapping at the heels of the LibDems for third place.

Except, that isn't quite true. As this YouGov poll demonstrates, the Greens are stuck on 3% (which isn't good enough to return any Assembly Members) while UKIP are the only likely challengers to the LibDems on 5%. Indeed, UKIP are polling ahead of the Greens and BNP combined.

You could argue that the Evening Standard are including the Green Party and not UKIP on their exclusive debate because the Greens won representation on the London Assembly last time. But if that's the criteria, why aren't the BNP included?

It really doesn't add up, does it?