I use t mobile's flex-t. I use it in a gprs router and they don't charge per mb etc like the rest... just thought you would like to know that.... they used to supply the gprs card on contract for free.... it costs quite a bit on payg.. i'd have a look on there website... I get unlimited gprs at 3g speeds through it... its fantastic for on the move broadband..
By the way... I used to work for a company who used to fit mobile broadband into police vehicles... so I know a little about this techi stuff :P