The reason? - Ah well - consider this from their constitution:
"The British National Party stands for the preservation of the national and ethnic character of the British people and is wholly opposed to any form of racial integration between British and non-European peoples. It is therefore committed to stemming and reversing the tide of non-white immigration and to restoring, by legal changes, negotiation and consent, the overwhelmingly white makeup of the British population that existed in Britain prior to 1948"