Some of the 12 wheeled cars built for the Caledonian Railway, and subsequently bought by the LMS for use principally in Scotland, lasted into the 1950's, but these would be in LMS and then BR livery. The 12 wheeled cars 'disappeared' from regular service on the LNER when the sets of all steel type K cars appeared in 1928 for the East Coast Pullman services. (Which won't stop this BR(ER) modeller from acquiring a couple; they can run on a race day special taking toffs to watch the gees.)
Their main haunt was the Southern, and they lasted in regular service on BR(SR) until about 1960. Up to that date they could be found in any loco-hauled Pullman service, as they were completely interchangeable with the 8 wheel cars. Just don't have more than one or two in a BR(SR) 14 car 'Brighton Belle' set, as the 8 wheel cars were much more numerous.