In the case of BT Broadband there is a "honeymoon" period until the Summer meaning they are not checking how much is being down/uploaded - thereafter they will provide customers with a tool to check their usage and the bandwidth limits will be enforced then.
If you go over the limit there is usually an option to buy more bandwidth, with associated costs attached.
Personally I wouldn't bother with the ones offering 1GB-2GB capping - after all plus.net and others do full speed/unrestricted bband for £18.99+VAT x month, which for the UK is not an unreasonable amount of money to pay - you know what you paying each month rather than having to guess/forecast. Having said that if your intention is to use for the odd email/casual browsing it might be useful - but you might catch the bug and be on it all the time. like me...
And these charges may well fall due to increased competition and the recent announcements regarding BT charges being lowered.
My 2 pennies worth anyway.
Charles