I assume that you are on Sky connect as you can use the BT speed tester website.
Unfortunately Sky connect has been vastly oversold for years now and some people have reported sub dial up speeds during peak times due to lack of capacity.
There is a nice FAQ here
The only real solution is for people to move to another ISP that don't oversell their product.
Edited by undecidedadrian (Tue 27-Sep-11 22:23:31)