I have always wondered this too.It would be nice to have some sort of "meter" which would show Internet usage.
BT DOES send you an email if you are near the limit,I had one today telling me I have used 7gb in Feb,but as you say it would be better to be able to monitor exactly how much you have used.
It is a free tool and will monitor all traffic up and down and organise it into different categories i.e. by day, week, month etc. It even forecasts usage based on previous usage. I used it for a long time before I switched to an unlimited plan. Note that it will only monitor per-machine bandwidth, so if there are other PCs in your house then they won't be taken into account, just the one it's installed on.
If you have google gadgets on your desktop, there are quite a few listed under technology to put on there. I also use netmeter (I think it's more accurate).