I love BBC Radio and I have been searching for the url address' for the radio streams since I got my N900. I only got it last week, so it has not been that long a search :-). Anyway I found this site which is aimed at iPhones:

http://bbcstreams.com/

I have added the links on there to mplayer and they have been working perfectly. I should point out that I have installed the extra codecs and so they may be needed, or maybe not. I'm sure a clever person here could confirm.

Cheers