Sometimes the best decision is no decision, I think. And this is one of those times possibly?
Boe, IMO the most important thing right now is that you get your daughter back. If there's even the remotest chance that this man will blame you for outing him and this in turn weakens your position in court or puts you in danger of getting your legs broken, then don't expose him until you have your daughter back.
If this man is exposed, it's human nature to suspect his enemies - those who hold something against him. You, therefore, are a prime target, even if the information is in the public domain and he can't prove it was you.
That doesn't mean you'll never expose him. It just means that now is not the right time to do it. I hate all that the BNP stands for too and those who fund it. I think it could serve a useful purpose to expose this man, if he has set himself up very publically as a pillar of the community. But for your daughter's sake IMO principles and revenge have to take a back seat for now.
Talking about revenge, and I'm not saying this is right but.....could you look on it this way: I know nothing about this man, haven't read the newspapers, but presumably he is on the ascendent? if he is not that well known, his fame is growing? if that's the case, and you wait and don't expose him till after your court case, it could have far more impact on him personally and publically because he will be more famous. If you want revenge, waiting will get you more of it. If you want anti BNP publicity then time it for maximum effect. How often does he give money to the party? do you know? if it is an annual donation and he has only recently made it, he will not be giving more for a good while. If you can wait and find out more, you can expose him better. Playing for time, not letting on what you know, might also give you a clearer picture of how much his wife agrees with his BNP support.
You have only just found out this information. Sit on it for now. If you then decide to expose him, you might be able to do more with the information than you first think.