You can get various different mobile broadband packages - the best offer that I have come across is by 3. At the end of each month they tend to do some good offers - last months offer was 5GB download limit for �7.50 per month - including free modem on an 18 month contract. Usual price is �15 per month.
Vodaphone also do a package at �15 per month, but you have to pay for a modem unless it is a 24 month contract and it is a 3GB allowance.
o2 have an offer of �20 per month and includes wifi, but it is only ope to 60% of the population - though this also comes with home broadband free.
Tmobile also offer some good deals.
Best thing is to go each operator's site and look up mobile broadband
If you are using more than this (eg downloading videos / music a lot) then mobile broadband is not for you.