Boris Berezovsky has declared his intention to violently overthrow the government of Russia.
The end for Russian President Vladimir Putin in that situation will not be a very good one. Quite frankly, if President Putin decides to go medieval on Berezovsky, it is self-defense as far as I am concerned.
The British have an obligation to not provide this loose cannon with a base of operations, as well. They need to hand Berezovsky over.