If you do not want to support the main parties, then make sure that the other minority parties get enough votes so that they are above the BNP in the allocation list.

For the sake of humanity - vote, know who you are voting for, what they really stand for, and why you are voting for them. If BNP still get seats, then we will know that there is a big problem with racism and neonaziism growing. If you are going to make a protest vote - don't waste it on the BNP.