I also seem to recall that you occasionally used to get all Mark 2 sets on the Manchesters (and maybe also on Liverpools and Glasgows). Not sure if there were any Euston to Manchester, Liverpool or Glasgow services that were booked for all Mark 2 sets or if it was just random what kind of stock you got.
Depends very much on the period. Pure Mk3 sets with Mk1 RKB/RBR and BG seemed to only last until 1977 once all the seated vehicles were in traffic, then sets became more mixed with the 2E/F vehicles.
Late-70s to 1987-8 (with the exception of Glasgow services from 1985), most sets on the Euston - Manchester/Liverpool/Glasgow workings were often booked mixed Mk2/Mk3 formations, even if just the odd Mk2F FO or TSO in a set.
In the mid-80s, other than the Pullmans sets were mostly made up according to necessary seating capacity for the diagram with some very mixed sets booked, particularly to Liverpool.
Then as the 80s went on, BR switched to more homogenised formations of all Mk2F or all Mk3 (bar catering and full brakes), in preparation for the introduction of push-pull working.
Mk2F sets (bar RFM and DVT/NHA) were booked a few Sunday duties off the EBW route, as well as the Carlisle semi-fast on weekdays.
They would also be stepped up as necessary, there are more than a handful of photos showing 2F sets on the Royal Scot for instance either in the 1990s under BR or right into Virgin days in the 2000s.