Whilst checking a few options recently I have seen some UK trains requiring reservations. The latest was when I was doing a check on a UK planner for the recent question on Holyhead to London. There are a few direct trains operated by Avanti and they clearly say a reservation is required to travel on those trains.
I have also seen some LNER trains with the same requirement on the Interrail planner.
Best answer by Al_GView original