before you think of changing ISP as anskyber said or similar to his check out click here for feedback on ISP ratings on what the general public think of BT's performance they are ranked at 36 so I wouldn't really recommend going with them, go for someone near the top ADSL24, Titan ADSL, who carry out 1 month Contracts, generous usage allowance and basically all the works.
Before you consider anything though what does Samknows tell you what you can get at your exchange??
I have used BT customer support in India and in UK, regarding my Broadband when it was first set-up and when I upgraded it from Option 1 to Option 3, had to hang around a bit on the phone but the end result was that everything went OK.
premier man: I was with BT up to August of last year and then became disillusioned with their speed - changed to Virgin. Had a 'run-in' with their Indian call Centre, couldn't understand a word they said and at 35p per minute that was a definate no-no! Very quickly returned to BT and am satisfied - getting, on average, 2.5megs which is all I can expect at my address.