Would the Alternative Vote be good for the BNP? No. Nothing could now be good for the BNP. Its vote halved between 2009 and 2010, when it lost 36 of the 38 council seats that it was contesting, including every seat in Barking & Dagenham, where it had thought that it was going to take control. AV offers the BNP nothing, anyway. Its candidates would be eliminated in the first round, and its voters would have expressed no second preference. Assuming that it still had any voters by 2015, that is.
Would the Alternative Vote be good for UKIP? No. A Yes vote in next year's referendum, long before any election pursuant to it were to be held, would greatly accelerate the secession of the already existing, and rapidly growing, right-wing party nominally still comprised of Conservative backbenchers. They would not join UKIP. Instead, they would expect UKIP to join them, as surely as the Social Democratic Alliance, and the comparable local bodies at Scunthorpe and Lincoln, were expected to join the SDP. Against, say, David Davis, someone like Nigel Farage could then expect to do about as well as Stephen Haseler did against Shirley Williams and Bill Rodgers. Meanwhile, the Old Labour half of UKIP's Strasbourg vote would have been thoroughly alienated, but right at the time when a major party representative of their views across the full range of issues had been called into being by electoral reform.
And would the Alternative Vote be good for the Lib Dems? No. That party, as such, is entirely a product of First Past The Post. It does not do very well for Strasbourg or for Holyrood. It does really rather badly for Cardiff and for the Greater London Assembly. And when its alleged Holy Grail, multimember STV, was introduced for local government in Scotland, then its number of Councillors there went down.
Bring on AV. Vote Yes.