Aquaplane:
>>Our ISDN 256kbit/s leased line here at work has 1:1 contention yet runs at half the speed of a regular cable line (512kbit/s). It costs in the region of £18'000 a year to have this service, so you really think it'll be as good through a £500 Cable Modem...?<<
The term "cost of ownership" comes in to play
This is why VPNs are so popular.
Telewest guarantee 256k download speeds and that is what you get, as well as an always on connection. You can always set up a web proxy server from another ISP to avoid out of date caches. I do not have any experience of problems with cache engines.
I have used Telewests Blueyonder for some time now and I think it is the Bees Knees for an "at home" connection.
And believe me I have seen some very poor connections in the leased line and isdn sector