Underground Model Railway

Abbey Road was a regular attender at the Easter Depot open weekend. Another possibility is Steve Smith's "London Road" which did have a large number of scratchbuilt ESLs, road and rail LT service vehicles and even the odd small CLR steam loco. Steve is/was on the staff at Neasden. Another possibility, which I never saw at Acton but certainly was on the model railway show circuit, was called Walford and again that employed LT road and rail vehicles.

Presumably the expense and lack of variety of the available stock.

Plus, of course, if you want to show their USP, namely that they run underground, you could only see them entering/waiting at/leaving stations.

Otherwise it's just a normal layout with underground stock.

Also, you can only realistically run very limited types of stock on underground lines, whereas most above ground lines could be utilised with quite a lot of different stock.

According to my research, the 1970's television celebrity Hughie Green had an enormous train set. None of it was kept Underground so he was able to find time during an overnight possession to produce the late Paula Yates. Mind the gap.......

There was a TV drama in the '70s featuring such a layout under somebody's floor. Can't remember anything else about it, though.
I'd like to see it again.

he had two flats in Chiltern Court beside Baker Street station - one to live in and one for the layout

That explains why he was the chairman of the Tenants association.

On a point of accuracy, pre-war CO/CP stock was always red. It was the similar-looking post-war R stock that came in white/silver as well as red - although I think by 1960 all R stock cars were silver.
(There were other differences, such as destination blinds as opposed to boards, internal lighting, number and size of windows etc)

O and P stock had all been converted from Metadyne to camshaft control (CO/CP) by about 1960. Until the arrival of the C stock, CO/CP stock was standard fare on all three routes (Circle, H&C, Edgware Rd-Wimbledon)
Most Q stock (Q23, Q27, Q31, Q35) had been converted from older G, K, L, or M/N stock on the late 1930s (mainly by fitting power-operated doors) and had various body styles. Only the new-build Q38 cars had the distinctive flared bodyside shared with O, P and, later, R stock. Most trains of Q stock were mixtures of styles - the one illustrating the Wikipedia entry is typical - and a train made up entirely of flared-body Q38 stock would be rare indeed, particularly after 1950 when most of the trailers had been converted to R stock. The last Q stock ran in 1971.
1971 was also the year that R stock was re-formed from a mixture of 6- and 8-car trains into standardised seven-car formations. These would have been too long to run on the Edgware Road branch and so would not have been seen at Notting Hill Gate after that date (and were, I believe, rare even before then)
As I mentioned above, all these stocks were originally red, but the R stock was painted silver in the late 1950s (or, in the case of the aliminium-bodied cars, not painted at all).

Also, some of those "flared" body sets which looked silver were actually painted gloss white rather than silver or the flat grey of the unpainted ally carriages.