This may somewhat of a dumb question, but how does the BNP enforce their "white-only" membership criteria? A quick scan of the BNP website revealed that I could hypothetically purchase a BNP membership card for £30.00 despite, well, being neither white nor British. If I had £30.00 to blow away, I might be somewhat tempted to see if I could beat the system.
The BNP candidate that was wrongly declared the winner in a suburban Birmingham ward back in 2006 was expelled from the party shortly after it emerged that her father was probably black. A total coincidence, I'm sure.