My take on each question is that, first, the Beeb aren't wrong to give the BNP a platform. Personally, I find the party's views and its politics repellent, but the fact remains that British voters have given them two seats in the European Parliament. An impartial broadcaster would be negligent if it didn't somehow take that into account.
And, as for the second question, the other parties should welcome this opportunity to take on Griffin and his ilk. The BNP exploits rational concerns about, say, unemployment and immigration, and twists them into irrational fears and hatreds. The other parties have to show that they're prepared to discuss those concerns, otherwise they're just establishing a space in which the BNP can continue to operate and thrive. There's no better forum for that than a TV debate; especially as the Griffin facade tends to crack in front of the cameras.