isn't this our current situation with sleepers in the UK? Edinburgh, Glasgow and Aberdeen have loads of flights from London (and Inverness has some), but there's also expensive sleepers...
Not really, as it’s more subtle. The sleepers here are very heavily subsidised for historical political reasons. Someone would have to do that for any sleeper to Europe, and there is zero appetite for that in the U.K. at least. To put this into context, the lowland sleeper (including the seats) has rather less than 1% of the London area - Central belt air/rail market. This market is more than double the size of the biggest international city pair for air travel in Europe, which is London - Amsterdam. There simply isn’t the market.
Any new international sleeper from the U.K. would also have to demonstrate that it would worthwhile capture market share from other modes, and not just transfer from day trains. Otherwise you would just be paying subsidy to move passengers from one train to another.
Finally, the sleepers here do actually run at times where they can capture market simll6 because of their timings. They notably either start or arrive (or both) at times when flying is not practical - if you are in central London after 1930 and want to be in central Glasgow or Edinburgh before 0830 the next morning the sleeper is your only choice. A U.K. sleeper to anywhere past 500 miles away (by rail) in Europe would be at least 12 hours, and at that time you are eating into the day considerably, and bumping up against flight times. For example, a Frankfurt sleeper would either have to leave too early (to make a breakfast arrival) or too late (to be later than the last flights and catch people from late meetings etc).
Oh and finally, finally, border control inbound. I’ll leave it there.