A London number at the time would be from the UK (01) eee nnnn where e is the exchange[1] and n is just a number, and the 01 can be omitted if you're in the 01 area. For intentional calls it would be (+44) 1 eee nnnn, as you omit the initial 0 that on an internal UK call is an indication you're dialling an STD[2] number and not a local number. There doesn't seem to have been an 831 exchange at the time[3], so as noted in the article the cut to shorten the number is presumably somewhere in the middle.

In 1990 London was split into 071 (central) and 081 (areas), which meant in 1995 all landlines could have an 01 prefix, so London because 0171 and 0181 (and that ruined the Live and Kicking phone number jingle, although I think the Going Live one was better anyway) and finally in 2000 London became 020, with the 7/8 moving to the first digit of the local number (not that the split between local and area codes really matters anymore).

[1] Of course, in ye olden days before all this newfangled "dialling" you'd ask the operator for something like "WHItehall 1212", not 944 1212 (or later 930 1212).

[2] Subscriber Trunk Dialling.

[3] https://rhaworth.net/phreak/tenp_01.php has a list, although it seems to be based on a 1968 publication.