"I've also personally encountered a few BNP supporters/Bnp-curious types too. A case in point - a previous work colleague of mine intends to vote BNP, on the grounds that they will endeavor to increase the state pension"
So it's ok to ignore the rest of their ideas because they have some good ones. Just a reminder, Hitler had some ideas that helped the Germans.. but there was some slightly bigger ideas that should possibly not have been put aside in favour of them.
"Anyone with "half a brain cell" should be aware that many muslims are incapable of terrorism but I would have thought that this logic could be applied, equally, to the BNPs electorate."
It's not a fair comparison. For one the Muslim faith does not seek to destroy all non-Muslims. And so the extremists are going against the grain. I think the best analogy of the terrorists is
"Extremists are to Islam, as the KKK is to Christianity"
On the other hand, the main theme for the BNP has always been "purification" of Britain, so really the non-racists are going against the grain. In my mind it's like someone saying "I'm a vegetarian but I eat meat" - it makes no sense.
"So please, lets not assume that the BNP have a monopoly on racism."
No one has assumed that, in fact, so far you are the only one to have said it.