"Neil Williams" wrote

there are
plenty of stations (Milton Keynes Central is one, Euston another)
where PFs apply to one TOC but not another. To determine what should
be charged, "sneaky" questions are usually asked.

If you're travelling between Victoria and Gatwick Airport, Southern trains
are subject to a penalty fare regime, but you can buy tickets onboard
Gatwick Express trains. Gatwick Express is now part of Southern.

Peter