Jon Cruddas has a good article on how the FPTP system allows the BNP to thrive in safe Labour etc. seats.
here.
But yes, the BNP might get seats under PR (proper PR, not AV). But I don't think that's a good reason to shy away from it.
Here in the Czech Republic there's a 5% threshold, but it contributes to a "wasted vote" thing so I'm not too keen on it. I.e. people don't know whether to vote Green in the forthcoming elections or not, since opinion polls show them at 4% and people don't know whether they'll cross the threshold. (They used to have more, and were in the last government).
(I'm not suggesting that any party on a couple of % should get in, but just saying that a % threshold doesn't work well with opinion polls).