- 5 minutes for disembarkation and platform to clear
- (25mins ex-Scotland, 20mins other services) delay recovery time
- 10 minutes for cleaners to clean with doors locked (platform called once doors locked)
- 7 minutes to board
So 42 to 47 minutes. Although this would require the way the station works to be overhauled into something far more suitable than that which exists at present.
To avoid overcrowding and the Euston Scrum, the circa. 15 mins allowed for boarding
has to be sacrosanct. I'm less fussy about the delay recovery time, but it depends how late you want departures off Euston to be. From a cursory examination of RTT, delays up to the 20 minute mark seem pretty common, so a 20 minute recovery time would be reasonable. If, however, you're happy for trains to routinely leave late with the consequent performance issues across NW&C and beyond, then you can trim that down to suit your taste.
A hot set would be useful, but I'm not sure I'd trust Avanti to do set swaps on such large numbers of services and have them ready to start to time whilst retaining the 15 minutes protected boarding window.