I remember doing a merrymaker excursion on an old set of southern region MK14 which had composite brake coach (forget configuration, might have been 2 fist compartments and 3 standard compartments, guard compartment, van) think these had originally been for portion trains like Atlantic coast Express, I believe some might have originally been tri-composites (first, second, third class for boat and named trains)
As built the 4CIG and 4BIG units had 4 compartments in their driving cars, all had one set up as 8 seat second class, and 7 as 6 seat first class. The first class was by the driving cabs and because of the side corridor to compartments there was an odd row of 4 seats backing onto compartment wall facing a partition. The other second class was open saloon.
The 4VEPs also had 4 compartments (but at non cab end) in each driving car, but as they had more seats, all 8 were First class
The class 442s as built had 6 First class compartments of 6 and a small standard class open area of 14 seats, so these also had composite vehicles. (this was fairly quickly converted to extra First class). From memory only 12 of 24 units were built with double guards van and composites, one of these were soon converted into extra seating to replace the lost standard class. Years later the other van area was converted to a snug