0800 isps are what they sound like, providers that used a freecall (typically 0800 prefix) number so you wouldn't pay by the minute.
Basically in the late 90s before all the mainstream flat rate services took off (starting with BT free weekends) there were some dodgy BT Internet accounts floating around, then a few providers started offering legit services but of course there would be hacked passwords etc doing the rounds. Also one quickly discovered that certain IP ranges on the BT free weekend often didn't cut you off after 2hrs like they were supposed to - if your hostname was of the format "host62-x-x-x.btinternet.com" you had a good chance of not getting booted (quite sad I can remember this I suppose!). So at 10mins to midnight on Sunday you'd redial as many times as required to get on that host. Then there would be the anxious wait to 01:50 to see if you'd stay online before calling it a night, downloads chugging away. Slowly throughout the monday people would drop off, but you'd get the odd lucky sod staying online for days. Other interesting vagaries of the system included the fact that despite BT prohibiting the use of dual-channel ISDN on the free weekend service, foolishly they'd only blocked multilink on the published ISDN number. If you dialled the v90/k56 modem number, it would still work on ISDN, and multilink was permitted!
There were even some really crazy ass ideas being introduced, like this company X-stream released a freecall isp using some custom software to display adverts and limit how many dial attempts you could use (it was massively oversubscribed). But it was trivial to just take the number out and manually create your own dialup connection using some utility to spam redial. Needless to say they didn't last very long, nor did ISPs like ezsurf and RedHotAnt that offered LIFETIME (or at least a year) unlimited access for a flat fee of say £25-£50. Essentially my interpretation is there was a MASSIVE flaw in market research they did, they looked at what the average amount of time people spent online was, it was maybe 5hrs per week. But the reason it was only 5hrs per week was because people had to pay by the minute! If you offered a free service, you'd instantly attract all the heavy users. Bear in mind this was dialup days i.e. a single channel ISDN could download about 500MB/day. So obviously you'd leave it downloading 24/7 to pull down stuff, it wasn't like today where people have connections capable of pulling down anything they want in under an hour.
Another 'scam' some people I knew in London pulled was there was this telco that didn't charge for local calls up to 10mins in length. So they'd tracked down an ISP (Nildram) that has a published localrate number and also offered static IP. Then one of the guys wrote a program to automatically redial the ISDN after 09:50 or so. Because ISDN reconnects so fast, and because they had static IPs, this basically meant that they could stay 'permanently' connected to 128k ISDN save for a couple of seconds of downtime every 10mins. They wouldn't even timeout from IRC etc because they'd come back online with the same IP before any timeout period.