To the point that when smoking was permitted you had:
1 Second Class loo
2 Second Class Non-smoking compartments
1 Second Class Smoking compartment
1 First Class Smoking compartment
1 First Class Non-Smoking compartment
1 First Class loo
Van space
Though, it would seem they were never used as the ultimate 'short train' in this fashion. Their main use originally seems to have been for portion working as a Composite brake and generally with a Second coach rather than alone.
Mind you, how about the early morning Aberystwyth-Shrewsbury train - Class 25, one CK and one BCK. Forget where I've seen that photo, might be a book but it struck me as a good prototype for a two-coach train in the early 1970s.
Or there's the Far North when Wick/Thurso each had a separate portion, BSOT+TSO to Thurso was the norm for a period in the 1980s. Wick got 4 coaches, the height of luxury!
It has often struck me as more interesting than surprising that two coaches seems to have been the minimum length under BR, I've yet to find a booked single coach working prototype. On the continent meanwhile I've found plenty of trains in the same period of the 1980s/90s where you did indeed have single coach loco hauled trains.
Perhaps the UK requirement for a "brake" vehicle robbed too much room for a single brake coach to suffice capacity wise, or that below two coaches a DMU was more economical?
Just some musings, apologies for going off on a bit of a tangent.
The Thurso portion of the 1980s provides a prototype for that
I would argue so, not to belittle such use - as always Rule 1 applies if desired - but as I outlined above, BCKs don't seen to have been used in such a fashion so far as I can tell.
Its an interloper
(Passports at the ready...
)