The absence of that service is an occasional nuisance to the, oh, six of us that would use it in any given year. That'll be me, @JamieL, and I'm guessing a handful of our colleagues who might deign to travel by train. And to the entire rest of the world, including the unfortunate staff who might be expected to work it, its absence is a blessed relief.
I was looking at some actual passenger numbers, and I think I may have been a little harsh.
The London-Inverness route, like the Bristol-Central Belt route, is technically possible by day train, but a pain in the backside. And there's competing air travel on both routes.
Combined air traffic from Glasgow and Edinburgh to Bristol is about twice the combined air traffic from Inverness to London. Looked at from the other side, Bristol-Edinburgh day train traffic is a bit more than
half of Inverness-London day train traffic.
That suggests that the potential market for a Bristol to Central Scotland sleeper train is comparable (within a factor of two or so) to that of the London to Inverness sleeper. Obviously anyone seriously proposing such a service would want to carry out a more detailed traffic study. And would need a serious pile of money they didn't care about seeing any return on. But there's at least some grounds to think there'd be a reasonable number of passengers.