I think its fair to say that Zen Internet probably provide one of the best IP STream based DSL services. Its not just about sync speed - that will no vary with ISP, thats a last mile/internal wiring thing. But Zen's network and peering seems to be more than capable of dealing with the capacity. The 'ancillaries' - mail/news etc is preety good for ISP freebies, their webspace is pretty featureless though.
Zen also have a UK tech help line, normally answered within 20s once through ACR. I have called them a number of times and cannot fault them.
Connection appears to be unshaped/traffic managed.
But if you don't like 'em, 30 day contract iirc. The biggest thing I can say is how many other ISP's DSL services could run a site like OOF?
BT's own broadband has improved massively recently, and TS better since bring back to UK. Obviously some people will not like the ad targetting system (which I believe Virgin had implemented anyway), so if that bothers you, BT are not for you.
Plusnet - great for your mother etc, with simple surfing needs. Very heavily traffic shaped, so pretty poor for anything else. Their own VOIP works, but 3rd party ones are problematic due to the shaping. Also have a bit of a reputation with email, after deleting 1000s of mailboxes 3 times in 1 year. Wholly owned by BT, so possibility of the ad tracking being implemented.
Tiscali have also improved, but the CS still lacks.
For best I would go with Zen, but you will pay more.
If the ad tracking doesn't bother you, BT worth a consideration.
For simple cheap surfing, I'd go with Plusnet, but probably use another email provider
Something in between is where things get tricky, and you get into that 'great when it works' type territory.