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Just need to remember Dun Run roads in and out of London during daylight hours are generally not so lovely.
You could bolt on a ride to and from one of the ACME 300s to make up a 400? Eg Green and Yellow Fields is flat, though I find Norfolk brings its own challenges and I prefer climbing :)
https://ridewithgps.com/trips/22923236
There's also a 400km version of the London orbital, the London circuit steam ride as well.
https://www.komoot.com/tour/760004634?ref=atd
I’m in the opposite boat, looking for a dead easy 400 near LDN for my maiden SR Series. Current options are a lightly extended Dun Run there and back, or a route called Around the Weald in A Day wot I found on RWGPS. Am erring towards Essex as I see a lot of Kent. Suggestions welcome.