Re: It was always fibre though, right?
Certainly, there was a time when most of the trunk or core network was fibre, while the analogue phone system beyond the local exchange was copper. That was the concept of System X.
I worked for a company that was providing BT with telephone exchanges in the late '80s, and they were definitely selling fibre trunk equipment then. According to Wikipedia, the last trunk connection in the UK was switched to fibre in 1990.