Might be worth a mention, but possibly worth checking that it is a 12 month contract BT are offering you. A friend as just been informed by BT, that BT are commencing or trying to increase contract times to 18 months minimum, so as to bring it in line with other ISP's like Talk Talk.
I have BT Broadband, and recently was upgraded to 8meg, only proviso was that I re-start the 12month countdown for my contract. Had no probs with the connection at all, worked well since (couple of months or so now). Bythor