Look, part of the BNP 'appeal' is 'making a stand' when people feel their vote would not count if voting for the big three two. Much Like the reasons why I have voted Green in the past. If we had PR I doubt I would have voted Green - and many of those stupid people would not vote BNP, because they would feel that their vote had weight.
If you are in a safe Lab seat for eg, and you are racist right wing, you may feel that voting BNP is the right thing for you to do, rather than 'waste' a vote on Con. However if you actually know that your Con vote has value, you will not vote BNP.
Translating yesterday's voter shares into PR seats is pointless, as millions would possibly have voted differently if we had PR. Many people vote tactically, for one thing.