Connecting with a data card can be horrendously expensive. Especially if you use it to the same extent as broadband. Even BT's offer of 19.99 for 1GB limited useage will seem astoiunding value in comparison. For 1GB you can pay up to £85 a month. You are also limited somewhat by the quality of the mobile phone signal for your chosen network. So it won't be good enough for anythig other than a few emails.
Sorry to be a party pooper, but all the networks charge silly amounts for this kind of access, but the reason is that it is quite power hungry for the mobile phone slots in a given area, and that if they were to be more widespread then availability of call slots would be fewer and far between, and lower in quality. Mobile datacards are intended more for business people on the move, and the prices reflect this.