At uni we had JANET (still around), at home technically I think the first ISP was Lineone (still around as TalkTalk) but that was only really to just get online via a CD, I then signed up Barrysworld (defunct) straight away. Also used Nildram (now TalkTalk) and Netgames (dead) as pay per minute ISPs.
Various freephone ISPs including BT Internet (still here), Ezsurf (can't remember the trading name, went bust, not surprising really with a £27 lifetime fee lol), a shared RedHotAnt account (also went bust, they were £50 for life iirc), Xstream (someone I knew made a little app to constantly redial on ISDN much faster than Windows DUN could, they are dead) and AOL. Back then around the turn of the millennium there was a crazy land grab with all sorts of ISPs offering silly freecall access, just a bonkers business model as it was attracting all the hardcore users.
When I finally got ADSL I went with Nildram, switched to Eclipse (now KCOM) just before I moved out of my dad's house as it was cheaper.
Since then I've had NTL (now Virgin, good speeds but bad for gaming), Sky (still here), Orange (woeful, now EE), Entanet (still around, had good tech support), Vivaciti (still around) and BT. At the moment I have both Virgin and Sky meaning I can have a dedicated FTTC line for gaming and also redundancy if the Virgin line goes down.