I'm not saying that Brits don't find American humour funny or vice versa; course they do. But there is undoubtedly a difference in styles, humour-wise; otherwise why the need to make The Office into an American version, for example?
Like I said with my first post, I just wasn't sure Russell Brand with his 'scruffy unkempt dandy' persona would necessarily appeal (which is actually, IMO, him taking the piss out of himself and a big part of his act - eg, the hair/mental illness comment). Could well be that I'm wrong...
Perhaps, as Ian says, it is actually more about a foreigner daring to say stuff about Americans than whether they found him funny...