Upminster (in the film) was converted c1971 and at that time the manual exchange in March 1971 had 7100 lines on it. It was converted to a Non Director crossbar exchange which had 7700 lines on it a year later. The only electronic exchanges at that time were the TXE2 (Telephone eXchange Electronic No 2).
As far as I can find out, only one manual exchange was ever converted direct from manual to electronic - a TXE2. That was 'Rhosllanerchrugog' a five position CBS2 manual exchange in a
private house in Rhos Notice 'DP 1' (Distribution Pole) in the back garden of the exchange house - grey on on the right of photo. It had 893 lines on it when it changed over to a
TXE2 - commissioned by guess who in 1972? The first large electronic exchange was Birmingham 'Rectory' - the first TXE4 which didn't come into service until 1976. Guess who was senior commissioning engineer there?
ex-GPO manual exchanges do survive - I have a couple. One is the CBS2 that was Braunton in Devon until late 1960's and the only surviving CS No 3 exchange out of the 475 which the GPO once had in service. It was a small 3 junctions + 20 lines (originally it had been a 3+5 when it was fitted in 1925 by David John McFarquhar the GPO engineer for the area. The operator in whose house it was, was a Miss Kathy Fullerton. Also got the centre suite from
the switchboard which appears throughout this film series - the last large manual board that will ever be built as the rest of the positions were scrapped after the second series. There was nothing in them anyway as they had been stripped to get them out from their 'bunker type' location. It was the original GPO 'auto-manual' switchboard at the then newly opened Portmadoc automatic exchange in 1943 as in the black and white picture.