Demon - nope, not with a barge pole! I have been using them commercially for a while now, bad service, high costs and generally unhelpful.
I use Freeserve and Freeola at home, both decent access rates (better than Demon) and both free!
ISDN - not at £20 a month! Plusnet in Sheffield will do you ADSL for that (if you have an unbundled local loop that is ADSL ready anyway!). BT are now running a scheme to find if people want the service, so if you can find the right bit of the web page, and get all your mates/neighbours to sign up as potentially interested, things may happen.
I hate computers btw.