Utilities Warehouse. The more you combine other utilities the cheaper it gets.
Broadband help line is based in UK (I think) and responds quickly and helpfully.
I switched to them for elec/gas years ago, and 2 years ago added telephone and ADSL. Download speed increased by factor of 4+ from previous ISP (Eclipse), although I can't see how that could happen on the same BT copper wire. But it did. And costs reduced by 50%.
By and large, UW is always cheaper than most/all suppliers, allowing for very short term lags in response, when comparing like with like.