Unfortunately of course, by taking this action of refusing to deliver a legitimate piece of political literature whilst delivering others has drawn attention to the fact, which has no doubt been used as positive propaganda by the BNP. I'd certainly suggest that they have grounds for filing a complaint on the grounds of discrimination.
I wish that these things were simply left for adults to choose, rather than someone (in this case the post office) feeling that they have to make a decision on our behalf not to give us the option. After all, for many people, the BNP literature would, in itself, have provided far more reason not to vote for them than anything else.
*shrugs* I guess we'll find out in a few days whether the anti-democracy stance of the post office worked or not.