I recently got a PLY» -PAD» which had only 2 First class carriages, mid morning Sunday so not normally very busy but this one was and there were some very unhappy First class passengers left without a seat - First class was full by Exeter.
Likely, perversely, to be more of a problem at weekends, where many upgrade to Weekend First, thus leaving those that have bought 1st Class struggling to get a 1st seat if joining in Devon. Suspect the demise of Weekend First once this shrinking exercise is complete - just relying on getting the 1st class Advance quota right....