An interesting theoretical exercise from a SE England point of view, but I know a lot of people living in Perthshire who are some of the most regular users of the Highland sleeper. They turn up at Blair Atholl, Pitlochry, Dunkeld, Perth or Gleneagles around 11pm and reliably find themselves at Euston at 8am. Any other method to get there would be crackers. Drive from Perth to London for an 8am start, you've got to be joking. Drive to Edinburgh Airport for 5am, is that much better?
I can't see why anyone as far up as Inverness would try to fly on an infrequent service, drive or take daytime trains.
Aberdeen, Edinburgh and Glasgow are a very different matter and air is king. But to make any comparison between rail or air from the Highlands, and DRIVING to London for a business trip for a day or so is nuts. You may be able to cover the ground more quickly than the sleeper driving like a loony with matchsticks under your eyelids but, apart from anything else, what would I do with my car for three days when I arrive in The Strand at 830am on a Tuesday?