@plugwash
I think what you may possibly be thinking of is that after the Polmont derailment in the 1980s (train hit a cow with Mk2 driving trailer the leading vehicle), passengers were not permitted in the leading vehicle if it was built to exceed 100mph. This is why Driving
Van Trailers were built for the West Coast (Mk3s for 110mph) and East Coast (Mk4s for 125mph - or even 140mph as envisaged!)
This was later relaxed (with Voyagers and Pendolinos being the first) to the first third of the leading car having no passenger seating, and then a significant number of rows of seats facing rearwards, with eventually the possibility of some forward facing seats further towards the rear of the vehicle.
Thameslink trains have some longitudinal tip-up seats, but in the middle of the train. Crossrail and London Overground trains have vast amounts of longitudinal seating throughout. All of these do not go over 100mph, so have never had to meet any restrictions of rearward facing seating only (which rules out longitudinal seats) in certain positions.