> have few friends working for ISPs, they say is VERY difficult to track/record/find anyone breaking rules
Not so sure about that. It's not difficult at all, ports can easily be monitored at local exchange level (from memory the incoming port can generate a trap when it reaches a certain figure or configured threshold). After all you have a fixed connection to a static port at the local exchange.....
The trick would be having an arrangement in place between BT and the ISPs to share the information, but this might be covered at a wholesale level.
In Australia, some of the services throttle back to 56k once you have hit your limit. This is generally enforced on a monthly basis though.