I've been somewhat distracted with other things for a few days, so, re the OP, a belated thanks to
BS for the elucidation. Further searching suggests it's not so much a law or statute, but rather a question of breaching the
Class Licence for the Running of Branch Telecommunication Systems.
I see that that the thread has evolved into an interesting discussion on the various methods of hacking the phone system, and I plead guilty to all the above.
Would you also take into consideration, m'lud, the following:
Coinbox off a UAX - dialling 0 for the operator would light a red lamp on the parent exchange switchboard. But, dial 9 for the parent, then the code back out to the UAX, then 0, and bingo, it's a white light - indicating an ordinary line. Place any call you like, and it was booked to the coinbox number.
And the easiest of all - placing a reverse charge call to a coinbox. This was subsequently scuppered when the pay on answer boxes produced a 'cuckoo' two tone sound to alert the operator.
There's no fun at all in System X.