It isn't an unreasonable suggestion, DB run some regular services at night in Germany and they tend to be fairly popular with interrail users.It was just a suggestion that an overnight service may be viable or a worthwhile expansion.
A direct link to Scotland at say 2330 on a Friday or Saturday night would probably be fairly popular
London - Scotland on a coach takes about 9 hours, alright for an overnight coach to arrive at around 7:30 am the next day. The train takes around 4h30 so it would have to run slower or arrive at poor hours. You'd also have to find drivers willing to do overnight runs, likely at higher pay. Higher pay for a longer time while having fewer passengers who pay less and the profit margin quickly gets a lot smaller.I'd have it leave a little later - maybe 0030 and timetable it to arrive in Edinburgh at 0700. Same in the other direction.
NR doesn't like overnight paths as it makes it difficult to get overnight possessions for maintenance. The Caledonian Sleeper goes via the ECML when the WCML is closed