Look at Life was a series of over 500 short films produced in the 1960's. One episode shows the UG substation below Soho Square. Interestingly it has a couple of rotary converters presumably for supplying DC to legacy plant in the area such as lifts in flats etc.
One interesting feature is the slip ring shorting gear in action, though I have never seen such a feature on a rotary converter. Presumably this is on the starting motor. However, I thought later machines were self starting and synchronising and didn't require the additional motor.
https://www.youtube.com/watch?v=i1kO1oTOgOE&t=34s starts about 1:20 in
Thoughts welcome
Clive