BOAC,
The 'law' refers to the BT line INTO and as FAR AS the master socket.
I suspect you'll find its up to and INCLUDING the master socket, which is otherwise known as NTE (Network Terminating Equipment) or NTP (Network Termination Point) depending on who you ask.
If, like me, you don't have a master socket but a conn box, then its your side of the conn strip, you can't touch BT's side.
The theory goes that the NTE provides BT with a known-good demarcation point for testing and by extension contractually defines a service as working and "in-spec". It also serves as protection for exchange-side equipment. Hence if you tamper with the cabling prior to the NTE, you could adversely impact BT's measurements (or ruin their exchange kit if you sent high voltage down the line due to some clumsy wiring).
Edit to add :
From BT residential customer terms and conditions.....
By service failure, we mean the continuous total loss of the facility to make or receive a phone call, or of any other facility we provide to you, due to a fault in any part of BT's network up to and including the main telephone socket for your property.
The guarantee does not apply if:
someone other than BT has caused the fault;
we ask to come onto your property and you do not allow us to; or
we reasonably ask for other help and you do not give it.
Hence, if you mess up through your tampering, BT could quite rightly refuse to fix it.