No, my observation is supply & demand 101. Demand exceeds supply at the current price
There are two solutions to that. Either increase price to reduce demand, or increase supply.
is there a signifiant increase in cost for each extra car added? Especially if they were couchette cars? An extra attendant at say £300 per trip, or £30 a cabin with one attendent per car. Some extra fuel, but surely not a great deal. Setting aside rolling stock lease costs that seems a no brainer,
I guess the larger problem is the platform space at Euston. You can't add 6 couchettes to the Highland sleeper, the platform isn't long enough.
For rolling stock cost -- trains carriages are very expensive. A sleeper carriage is used for about 60 hours a week, or about 1/3rd of the time. I guess trains are just too quick / UK too small and dense for many 18 hour journeys.
Are European couchettes used for separate day services?