Just to add to the thread, I'm using ISDN (Home Highway) with Demon's unlimited access dialup account and BT's 24x7 surftime product.
Works out as about 55GBP/month for the BT part plus another 20GPB for Demon (inc. VAT). Not 'cheap', but certainly a lot more affordable than it used to be.
Demon's service has been pretty good, aside from the occasional glitch: Static IP, no 2 hour kick nonsense, a network status page that's actually updated, good performance, etc. The main problem I have with it is that 128k ISDN does not work with Surftime, but that's a BT problem rather than a Demon one.
Demon do, apparently, have a 'network' dialup package which works with dual channel and gives you a range of IPs, but the price goes up to 80GBP/month or so. Easier just to dial in on a non-0844 number to get dual channel when you need the extra bandwidth.
Hope that helps someone ;-)
Gareth.