I recommend T-mobile, very good 3g coverage in the uk, I have used web & Walk with a dongle before moving my contract from vodafone and while t-mobile do throttle your speed if you go over the fair usage, fair usage on web & walk used to be 3gb but might be 5gb now... the good thing about t-mobile is they never charge you if you go over the fair usage limit. web & walk is the package that comes with n900 (same as dongle package for a £5, but my web & walk package was free, on £20 month contract, 300 miunutes with 300 or 600 texts...).

Not only i manage to get the n900 cheaper then anywhere else, vodafone gave me an extremely hard time when i tried to leave and insulted my intellengence (as a long term customer) with an offer which was nowhere as good as t-mobile.