I was one of the original customers, via the CIX forum. I sweet-talked Cliff into letting me have a nine character hostname, though the usual limit was eight, so that I could be stonewall.demon.co.uk
For years, I think I caused immense frustration to various Demon engineers who had to put special cases into bits of code to cope with the anomaly.
Back at the start, it was a very geeky thing to set up - using things like ka9q or Trumpet on a PC, or dicking around with PPP and SLIP on Linux/UNIX systems, which were largely new to people in the UK at the time; before then it was BBS, systems like Cix, Compuserve, or a hookup to UKnet's UUCP service - which I think still charged 2p per kb for incoming international email back then.
So a direct TCP/IP connection was pretty revolutionary. Many, of course, came along and tried to imitate - not least Pipex, from whom Cliff got his first leased line. And yes, probably someone else would have set up a consumer ISP in time.
But Cliff was the first here; both that, and the way he raised the funds to make it happen make him a very significant person in the UK's net history.