The manifesto is interesting, though a bit woolly. I don't think they go far enough in part 1, which discusses partnership rights. I personally think all of the rights of marriage should be available to any group of unrelated people who want to make that commitment. But then I'm a polyamory activist, so that's not really surprising.
The one thing I note is that they talk about "heterosexual and homosexual relationships". On soc.bi, you get jumped on and flamed if you dare talk about a "heterosexual relationship" - it's an "opposite-sex relationship" (and likewise it's not a "homosexual relationship", it's a "same-sex relationship"). Some people on soc.bi claim that using "same-sex" and "opposite-sex" is more bisexual-inclusive. I'm not sure that it is, but so far, I haven't been able to think of a better alternative.
The most queer-friendly party fielding candidates in my area are the Liberal Democrats, who are running a campaign called Freedom to be... - basically standing up for the rights of gay, lesbian, bisexual and transgendered people. Our current MP is a Lib Dem, and I think she's done OK for us, so we'll probably be voting for her. Richard wanted to vote Green, but he disagrees strongly with their policy on the Euro, so we don't have a lot of choice really :)