Most ADSL suppliers will be shaping, have terms and conditions and caps attached.
Zen have long been a bastion of uncapped products, even they are admitting that 8Mbit uncapped isn't viable at a half-decent price.
http://www.zen.co.uk/about.aspx?page=106
Also you think that the speeds on ntl 10Mbit are variable I can assure you that you'll see a lot of instability on BT's offerings due to contention as well.