I will move home shortly. I wanted to move all BT services to my new flat, but, unfortunately, the transfer is much worse , not between 5 - 12 Mb but between 1-3 Mb! So, I have decided to terminate a contract with BT.
If you're still in a minimum contract period, then there's a cancellation fee to pay. If not just ring 150 and select the "cancel" option on the menu.
If you are in contract, then there may be a free get-out coming within the next month. Read your bills carefully, including anything that comes with them. When BT announce a price rise (which they are about to do), then you have 10 days to write to them to cancel without penalty.