They do ask you not to give blood if within the last twelve months you have had sex with a man who has had oral or anal sex with another man.
You also can't give blood if you have had a blood transfusion since 1980 (OK, that one's because of CJD, which they can't screen for, so a bit panicky) or if you have ever injected drugs (even once, no matter how long ago and no matter how many HIV tests you've passed with flying colours since then). I don't think it's homophobia any more than it's druggophobia. Just statistically certain groups are more at risk.
(i.e. I do think it's probably a misguided policy, but I don't think it's motivated by homophobia IYSWIM, just general paranoia)
Try going to America -- there you won't be able to donate blood at all because you're British...