As the BNP are an elected political party, apparently they are subject to the Race Relations Act and cannot discriminate membership to the party by race or skin colour. The whole thing is likely to go court.

http://www.guardian.co.uk/politics/2009/jun/23/bnp-membership-policies-legal-threat

Will this mean that in the future, I will hypothetically be able to join the BNP and take them down from the inside*?
*I don't know much about the topic, there may be other reasons they could prevent me from becoming a member