Have a look at the T-mobile site. They often offer sim cards with 12 months "free" internet but only for certain devices. I'm not yet sure what you need to do to get the sim cards to work in devices other than those in the current T-mobile promotion but you can let someone else do whatever is necessary and then buy the sim card on eBay for £15 - £20. Just search eBay for "12 months t-mobile" or something similar (occasionally the promotions are for 6 months). The "free" internet gives an unlimited allowance for browsing and emails but there's a ½Gb monthly limit for downloading films and music.

If you have a few devices requiring an internet connection in the same house, you should consider one internet connection and a wifi/mifi router. Many devices will actually work better if you put one mobile broadband dongle and mifi router high up near a window and then use wifi for everything. The T-mobile sim card mentioned above will do this for you in an unlocked dongle.