Ah now its all making sense, BT's acceptable speeds for broadband are 20% of your paid for bandwidth, so if you've got a 1meg connection its still acceptable to only have a 200k download speed so even if they throttle you to 25% of your speed you're still within BT's limits. Very sneaky. If you've got a problem though I'd say get in touch with the ISPA, they do get things done.
http://www.ispa.org.uk/