https://www.bbceng.info/Technical%20...digits-fm.html
^^^
On the 14th of September 1972 the link from Broadcasting House to the Wrotham transmitter switched to the new BBC PCM (Pulse Code Modulation) system.
I had read about the BBC using PCM for national FM audio distribution (funnily, when the CD appeared in ~1982, some complained that 16 bits wasn't enough, they were reminded that they had been listening to 13 bit PCM since ~1972)
Kirk Bayne