I kept a log of what worked what over a few weeks last year. Yes, very sad.
Anyway, normally the same pair of 92s would stick to the Lowlanders all week between Euston and Glasgow, with the same loco on the Edinburgh portion all week, and the same loco on the ECS in and out of Glasgow. Likewise the same pair of 92s would be on the Highlanders all week. North of Edinburgh was the same, swapping mid-week was uncommon.
Of course, this isn't 100%. Locos fail, need to go for exam, etc. But in a normal week there are only 5 92s working passenger.
So if you travel from Inverness to London one night and then back the next you would probably get the same locos both ways.