Both parties are right wing, supporting the class sytem and money. However:
The Conservatives look to global investment. They are not interested in the 'nation' as such, only the part of the population who have investment in international companies. The BNP are British Nationalists. They want to boost British-based companies.
The Conservatives want to stay in NATO and the EU, whereas the BNP want Britain to go it alone.
The Conservatives do not want to repeal anti-racist laws or seriously stop immigration, the BNP want to do these things.
The Conservatives are not racist. Membership of the party includes black members. The Conservatives support the monetarist system and invite all races to join the chase and become millionaire business people. The BNP are racist and do not want non-white membership, nor do they want non-white millionaires or bosses.