I can. It's because the London numbers changed from 071 and 081 to 0171
and 0181 at the time they introduced area codes starting 01. Had they
gone straight to 020 at that time, much of the confusion would never
have happened.

I'd shift the problem back even further. Going from 01 to 071 and 081 was
the short sighted move for me. I was at UCL at the time and they went from
having a single telephone number to direct dialling of all extensions at
around that time. So I was underwhelmed at the boasts of a doubling of the
numbering space when just one institution was now using about 10,000 times
the numbers it was. Was there a technical reason why eight digit numbers
couldn't have been introduced back then and saved us all a lot of hassle?