One other thing I'd like to add is the departure times of the Northern Sleepers. Way too early to my mind. Why a later departure from Euston is not possible is beyond me. 23:00 - 23:30.
I'd rather they didn't. The Fort William sleeper in particular does not arrive at its' destination until 09:55, and a departure time from London such as you propose would result in a lunchtime arrival. The arrivals of the other two portions to Inverness and Aberdeen respectively are also currently timed to allow passengers to make the most of a full day at their destination, and in the case of the Inverness train to connect with the morning train to Kyle of Lochalsh, or the Citylink bus that performs the same function.
Therefore I think that the current timings of the Highland Sleeper are optimal, and I'd be astonished if there was much of a market for those wanting to "take in a show" in London before heading to the Highlands. Personally when I use the Highland sleeper I want to get to my destination as soon as possible so that I can make the best use of my day at journey's end, and I would imagine that most are the same.
Plus, an earlier departure means that passengers are more likely to make use of the lounge car (and for a longer period of time) in the evening for drinks and meals: A handy bit of additional revenue for Caledonian Sleeper.
As
Neil Williams has said, while the pricing might not be optimal there is the option of connecting into a morning northbound service out of the Lowland sleeper for those whose arrival time is not time dependent.