Couldn't agree more about how good Demon were ten years ago, but then they were a small privately owned company, and reputed to be 'the techies' ISP', as they seemed to pitch at people with good technical skills and accrodingly employed knowledgeable support staff. However, the company's been sold/merged/taken over several times and there really seems to be no remnant of the original ISP left.
Leaving Demon, unfortunately, is a big upheaval as we run a popular (non-commercial) website, which orignally had a demon domain name. We changed to a .com domain name seven or eight years ago, and the website's no longer even hosted by demon, but there's still an awful lot of links to that old demon site, which just redirects to the new .com. And, of course, there's still the occasional (valuable) email that arrives addressed to the old demon email addresses.