plus its breaking teh T&Cs of teh supplier
the big thing is to make sure you get a good signal on whatever network you go with at the places that you need it(home, work, etc). Be warned that a good voice signal does not equal a good data signal.
depending on your mobile contract, with Voda you can get 500mb for 5 quid a month contract or 7.50 a month as and when and use your phone as a modem(phone dependant but most modern phones will do it). T mobile do web n walk for 7.50 for 1 gig but it seems they let you go to 3 gig occasionally(according to me colleague here). not sure about Orange or O2