fair usage is the big issue with the big five UK networks:

Vodafone: 500MB per month before extra costs. good 3G coverage.
T-Mobile: 500MB per month before extra costs. great 3G coverage.
Orange: 750MB per month before extra costs. good 3G coverage.
O2: No limit. Will cap speed if above 1GB per month good 3G coverage.
Three: No limit. No cap but poor coverage

Beware that O2 expressly forbid video and audio streaming on their 3G network. so that means no skype calls or youtube on the go.

Also note that iPlayer only works with the 3G networks of Vodafone and Three at the moment. But since the N900 is crap at iPlayer, thats negligable. and when they do release flash for the N900, you can always use wifi regardless of network.

Three allow audio streaming including Skype but you have to pay extra for that service on top of mobile internet service

Also if you want home broadband as well, most of the mobile networks discount their home packages if you are a mobile pay monthly customer. For example, my O2 package includes up to 20meg unlimited ADSL broadband for £7.50 per month because I am on O2 mobile, which is half price.

I'm happy with O2 as I don't use video or audio streaming on the go, only from my home wifi, but the advantages and disadvantages of each are something that you need to decide for yourself.

Other networks that I don't know much about (to research) are Tesco Mobile and Virgin Mobile. These use the O2 and T-mobile transmitters respectively, so their coverage should be good and great, respectively.