If using Wi-Fi, make sure it is secure if it is already then change your wi0fi password anyway.
Since I live in an area that does not get BT
I do not know much about them with respect to broadband, but they should show your usage in your account page at their website (any decent ISP should).
Also give them a ring and ask if they can tell you what is eating up the bandwidth.
If you do nothing more than surfing the web and reading emails then you should be nowhere near the 10GB limit. If you use iPlayer/4OD or similar setups and actually down the programs, then you are also uploading even if you do not know it, though unless you grab quite a few programs even with the upload it would be hard to reach. Most ISP I know of include uploads as part of the overall bandwidth used. Either way BT should be able to tell you what is using it up mainly.
Also do a sweep for a Virus using Anti-Virus program such as AVAST Home Edition (free) just in case.