The worst I've found is on sub-urban commuter routes where very few of
the regular stock have first class but occasionally one gets put on.
Naturally hardly anyone's got a first class ticket and the result is
either have everyone squeeze into standard class like sardines or some
have to go in first.

You might like to check with your TOC - my line (FCC) regard it as first
class if it's advertised in the timetable, otherwise it's usable by anyone.
Anyone holding a first class ticket wouldn't aim to board such a train
anyway (though it's probably a TOC specific thing).

I regularly used the one train in the morning and two in the evening which
are formed of 317 units carrying first class accomodation which were not
timetable-advertised with no issues, it was generally the difference
between getting a seat and not getting a seat.

There is sometimes a discussion with the inexperienced RPI's but once
they've checked it out they politely come back and accept standard tickets
(with the "ooh I've just come" look wiped from their faces).

Regards,
Dave