As Peter Dawe says first ISP was Uni Kent, then PIPEX, then (maybe IBM PC UG) then Demon in June 1992. I was part of the original group that went to the UKUUG at Farnborough and discussed how an ISP could be built and then Cliff went away and built it.
I pre-registered to be a customer, but my credit card expired by the time they launched, so was eventually customer number 63 (188.8.131.52 marvin.demon.co.uk).
I eventually joined Demon in '95 and left 10 years later.
It's a shame that Demon is now finally being closed.