Not suitable for road bikes and gets pretty rough and muddy around Napton.
I road home from a beer festival from Hackney to Coventry earlier in the year and the route was great.
I used cyclestreets.net and used the ‘quietest’ setting to get out of London. It was probably too wiggly a route to do with a group, but the ‘balanced’ setting is usually pretty good at avoiding really busy roads.
Link to my route here:
https://www.strava.com/activities/677166611
I think you’ll be doing well to fit it in in 100 miles.
I stopped at Stowe NT property, nice cafe and you don’t need to be a member to get into it.