It’s an expensive given, once you include things like securing a long platform at Edinburgh and bringing up French border control, and not permitting stops before a night-time staffed St Pancras or Ebbsfleet.Oh of course, that's a given!
I don’t think we will see staff present passports of sleeping passengers ever again in this non-EU country. It’ll be a Lille-style shuffle. At that point you might as well send the sleeper to Kings Cross to meet the regular Eurostar. Or that old forum favourite, an assisted journey from Euston.On a sleeper (which is not so time critical), provided the authorities were in agreement and the timings permitted it, there could be a prolonged stop in the secure area at Coquelles at around 22.00 while all UK immigration checks were done on board- with undesirables removed there. Don’t think Schengen has tge same hang-ups that we do so checks could be done on the southbound in the same way as was traditionally done on international sleepers. Collect passports from sleeper passengers in advance andxreturn them in the morning. Wake everyone else up!