You could go London - Inverness/Aberdeen overnight the first day, then travel across to the other city to head back to London, that will get you at least 2 of the diesel locos (just beware the Inverness part is still a hired-in 67 [unless that has finally gone back to DB with the 73s taking over].
The Fort William portion is open to the general public west of Glasgow so that is a slightly easier way to get another loco.
Electrics wise, if I'm counting correctly there is normally 5 in use each 'night' [1x Euston - Glasgow lowlander, 1x Glasgow - Euston lowlander, 1x Euston - Edinburgh highlander, 1x Edinburgh - Euston highlander, 1x Edinburgh - Carstairs - Edinburgh lowlander], so the easiest way to get more bang for you buck is to book on the Edinburgh lowlander train.