The Marylebone 115 cars appear in many films and TV programmes as this was long the "quiet" London station, especially off-peak, that suited this. You would never know this nowadays. The BR headquarters building next door, where the PR department was, gave an added bonus.
Marylebone and its suburban lines were actually notably isolated from the rest of the BR network, and the Class 115 units, once steam ended there in 1966, ran on for the best part of a quarter century very much in isolation, and really the only type of rolling stock normally in use there at all.
Just in passing, regarding the 108s, Derby works ran two dmu production lines in parallel right through the classic dmu construction era, the "lightweight" cars (108s etc), short frame cars with alloy bodies and low density seating/doors, and the "heavyweight" cars (115s etc), long frame cars with steel bodies, many (but not all) with high density body designs. The front ends were standardised, and when they changed over from the plain roof dome with high destination to the 4-character display with destination in the middle window, this was done for both lines at the same time.