The Sundays only Inverness to London Euston sleeper normally runs via the West Coast Main Line, with set-down stops at Preston and Crewe, and as such Perth to London Euston would not normally be valid.
However, official published timetables for the Caledonian Sleeper include the Sundays-only alterations for engineering works, which show the Sleeper running non-stop from Perth to London Euston.
Therefore the move is valid, and scores a Long Wait Bonus.