So here’s the facts.
Short version:
If you are travelling to lowland Scotland you can go:
Very cheap and uncomfortable for 8-10 hours (coach)
Cheap and uncomfortable for 6-7 hours (overnight train seats)
Cheap and uncomfortable for 3 hours (off peak plane and terminals)
Reasonable and comfortable for 4-5 hours (off peak day train)
Expensive and uncomfortable for 3 hours (peak plane and terminals)
Expensive and comfortable for 4-5 hours (peak day train)
Expensive and comfortable (although option is divided on that) overnight (Sleeper).
Which market segment is this proposal aiming for?
Long version:
The entire Anglo-Scots overnight public transport market has about 500-700 people travelling each way per night. This is all cross border, not just from London. Split is about 70% rail, the rest by coach.
This compares to around 4000-6000 per day on day trains just between London and Scotland (total both ways), and 11000-13000 / day by air to/from London. Daytime coach travel is peanuts by comparison.
Thus in a total public transport market of around 16,000 to 20,000 trips each way per day, the overnight rail market is about 2%, and the overnight coach market 1%.
The overnight seated rail market already exists. It is also reasonably cheap - generally cheaper than the day train, and much cheaper than a sleeper berth. The seats are rarely full. You can buy tickets for tomorrow for £60. It is therefore reasonable to assume that there is not a great untapped market for London-Scotland seated passengers from those who already use the train. People are evidently more than happy to pay more - much more in some cases - to travel in day time or in a bed.
So where could these extra passengers come from?
Air? Very unlikely. Fares are reasonably cheap, usually cheaper than the train. (You can go tomorrow for £40). People fly because it is (usually) cheap and quick, which outweighs the inconvenience and cost of getting to and from an airport. Particularly if your origin and/or destination is not a city centre.
Coach? It’s a small market, a handful of coaches each way per day, roughly split between day and night services. It’s the cheapest way, presumably because being on a coach for 8-10 hours is not a nice thing to do. The day fares are broadly the same as the night fares, so it is reasonable to assume that daytime passengers are travelling because that is the right time for them, not because of fare differentials. Thus the only available market is the night coach passengers.
Even if every single overnight England to Scotland coach passenger transferred to the train, it wouldn’t fill 3 Mark IIIs per direction. And to win them, you would have to at least get close to the coach fares.
Low passenger numbers and low fares does not a viable service make.