I know your pain!
Was on old Blueyonder 2MB and got consistant average of 250kb/s downloads, then I moved into my girlfriends and started using her brothers 4MB Blueyonder connection and got on average 30kb/s download rate!
After constant telephone calls to tech support with no better results I gave up as we were moving in a few months anyway.
Just moved downstairs in same block of flats, signed up to 2MB Blueyonder service again. Guess what? 250kb/s download rates back
There is obviously a problem with services above 2MB that needs addressing by Telewest.