Go with Be (or O2 who use the same network). I have been with them for four years now and get 14Mbps reliably with them.

If you can get the current attenuation statistics from your router, then this graph will show you the speeds you can expect to get when you change from the Max product (ADSL - Upto 8Mbps) to Total (ADSL2+ - Upto 20/24Mbps)

The speeds you get depend on the quality of all the connections between you and the exchange. I used to get 18Mbps, but then BT did some work on the cables in the pavement and my speeds dropped to 14Mbps. Since BT only guarantee you a voice quality line, there is nothing you can do about this.

To find out which exhange you are on, try Samknows Broadband Checker or Exchange Search

We could be in for a further boost in broadband speeds with FTTC scheduled for December (FTTC - Fibre to the Cabinet, upto 40/100Mbps) :D