Yes, there should be some way to express that, and that's as good a way as any. I don't think it's a useful opinion to express, since it sends the message to anyone who voted for the BNP that even if they go with the rules of the system, the system is going to ignore them. And it strengthens the BNP's case amongst its supporters as a result.

But if it's the opinion that you want to express, that's certainly one way to do it, and I agree you should be allowed to.