LOL I remember doing that on Sundays.
Another scam was war dialing through 0800 (you'd find BT numbers that way) for meridian PBXs, then you just press 9 and it gives you a line out, then you dialed an ISP number and it was free internet. There was a newsgroup called alt.ph.uk that was quite fun back then.
You could also get free mobile calls with those chipped BT mobiles, and free Telewest cable with a cable cube before everything was digital. There was a huge black market for all this stuff back then.
Then when Telewest started doing cable internet you could setup your own TFTP server with a modified config file and set your own speed caps. It would max out at about 6 mbit down, 1mbit up... while everyone else had 0.5mbit down. Then about a year later lol they sent out threatening letters telling people to stop it.
There was some extremely more shady stuff as well that I wont go in to.