I'm becoming quite attracted to the idea of bumping it up to 1900km, and getting the extra time allowance - 10kph - for the ECE bit. Front-load it, and you've got three days before your start time to do 500km (well, 73 hours and change).
The Mall and my home are each about 15 miles from the school - call it 25km, so with riding up on the Saturday for registration, that's 75km. Riding home at the end of the jaunt is another 25. Ride London is 160.
Assuming a Sunday pm start - would have to be 1600 I think. Do 250km on the Friday (or at any rate starting after 1600 on Thursday), have all day Sat for registration, then Ride London on the Sunday morning. Would need to be back at the Mall for 1400 in order to be reasonably comfortable getting up to Loughton.
Hmm. No way at all to be even remotely sure of that, is there? Start times for RL are between 0545 and 0900. A couple of years ago I had an 0639 start and finished about half one - that would work OK, but you don't get start times until early July.