You're kind of missing my point. The two are not comparable because..
a) the BNP have always been associated with racism. That has been their main characteristic for as long as I can remember, and I'm sure beyond.
b) The Muslim faith's main characteristic has always been belief in Allah and the main tenets of the religion.
With me so far?
Therefore the majority of people join the BNP due to it's racist characteristics. And the majority of people join Islam because of the belief in Allah and the main tenets of the religion.
So for someone to join the Islamic faith for reasons not in harmony with the main teachings of the religion, eg. terrorism, that would be going against the grain. Agree?
And by the same logic, most people who join the BNP are doing so based on it's most well known characteristic. So it's highly likely that most BNP supporters will agree with the main characteristic.
Hence my comment about vegetarians. The main characteristic is a belief that eating meat is bad for whatever reason, therefore you'd have to be a bit strange to say you are a vegetarian despite believing the opposite.
It's stereotyping in as much as saying someone who joins a church is a Christian, true there may be some exceptions, but for the majority it is true. And a non-Christian who joins a church wouldn't then be offended by being called a Christian, seeing as they choose to surroud themselves with Christians.