I don’t know what happened to them as they were not that popular in very busy trains as less seating in them. I guess they were ideal on the Midland Scot and Liverpool Scot service as only a few places would be laid up as you say.
I travelled on one in 1984 on a service from Liverpool to Reading but I'm sure no tables were laid up. More recently in 2012 I travelled on one on the Jacobite to Mallaig with a first class ticket, maybe it was substituting for a Mk1 FO. According to the RCTS list, WCRC has 5238 and 5254 in their fleet.