deakink--You might try
http://www.dslreports.com/tweaks
However, I do not know if you will have problems running the diagnostic from the UK.
There is also this site.
http://www.broadbandreports.com/faq/tweaks
DrTCP is often very helpful. As is the advice under #5 RWIN.
If you are being charged for faster speed than you are getting, BT really should try to help. At minimum they should come out to test your line.
Do you have any alternatives if they are not?