I've taken sleeper train services before from Scotland to London. It's cheaper than a hotel and the service leaves at 11pm and arrives at 7am. You get a room with a real bed (and lockable door, sink, toilet, etc.) and a pretty decent restaurant and bar to avail yourself of. You also get to leave from the city center and arrive in the city center, without messing around with security and arriving 1-2 hours before the flight.