You might need to get it unlocked if it's on another network. Most pay as you go services now max out at £1 per day for GPRS – but that's still £30 per month if you are going to use it for email.
If you want to use the BlackBerry email service you will have to subscribe to that as well (about £10 per month I think). Best call your network and see what they can do for you.
If you just want to use it for calls and texts you'll be fine.