You pays, yer money and you takes yer choice!
Am with Pipex, it is ok. Am happy enought - ecstatic at the speed of adsl
Speed is good, does have a few quirky moments (but don't we all). My only problem with them is that the help desk is only open business hours Monday to Friday. If you have a problem at the weekend you have to wait till Monday.
Oddly enough any problems I have had (not been able to connect, speed, etc) and there have only been three up till now have all been at a weekend. Then even more oddly, by Monday they have cleared up.
Have not used any of the others so cannot comment on them.
I think you will find from your choice that over a period of time they are nearly all on a par with each other. One month one is better, another someone else.