Slightly envious of that. I've never seen more than 2.5Mb on O2, and that was in central London. In most places I don't get more than 1.5.
In Norwich, I found O2's 3G network to be very slow and probably very congested. Whereas with 3, having mobile signal nearly always means having fast Internet access, which is great for streaming audio.
I use about 6 - 8 GB /month through streaming audio and the network just handles it. 
EDIT: Oops, didn't mean to double post.
