I would like to suggest the following as a pack of DLC... Western and Midland Blue Pullman sets. Mk2 Blue Pullman coaches. Blue Pullman HST set. Brighton Belle EMU set. Just imagine the lush interiors and unique styling.
The Manchester Pullman, which ran from Manchester Piccadilly to London Euston would be a great choice. We haven't got any UK 25kv electric routes yet, Crewe to Manchester in the 70s or 80s would be good and so many opportunities to use our existing diesel locomotives in scenario planner.
Yes, mk2s, but purpose built for this service and carrying a reversed BR grey/blue livery. Vacuum braked only according to Wikipedia so couldn't be hauled by 87s. https://en.m.wikipedia.org/wiki/Manchester_Pullman