Could it be done as two 75-mile rides with an overnight piss-up in between?
Mmm - there's an idea - although the big snag with more than a handful of riders on an M25 ride anyway on my original route is the QEII Bridge at Dartford, as the muppets there can't handle many bikes. Three in 1 go is the maximum, plus during the day there's no service between 9 am to 10:30 am and 2 pm to 3 pm.
An option would be to add about 5 miles on to use the Tilbury to Gravesend ferry. Have the ride start somewhere else with a decent train service which can handle lots of bikes, and the same, 75 miles further on. It has possibilities..........