Norm Geras has been running a series of posts about how Greens are instinctively anti-Israel. There's further evidence today that anti-Semitism is now a left-wing thing as the Scottish Green Party launches its manifesto and, on page 25, announces that it will seek to boycott Israel but mentions no other countries as being problematic.
Greens are fanatics unable to think clearly and are therefore dangerous. They're also London. UKIP (who are ridiculous but less so than the Greens) are Not-London. This is why the BBC - being London - thinks that UKIP is a small bad party in the same bad, Not-London category as the BNP, but the Greens are in a different, essentially good, London category.
As Daniel Hannan observes, you will frequently hear BBC reporters make throwaway remarks that are variations on: "...minority parties like UKIP and the BNP", but never "...like the Greens and BNP".
This is odd considering that the Greens and the BNP have similar support in most opinion polls (4%) whereas UKIP is at 8%; and at the 2009 European elections, UKIP came second behind the Tories with 16.5%, whereas the BNP (6.2%) and Greens(8.6%) got two seats each, making them very comparable 'minority parties', in that respect at least.