I can understand not wanting to ride out through some grim testicle of suburbia to get to the start of an event. I frequently hop on the train for a 20 minute ride out into semi-rural Kent to avoid having to ride through the SE London/Kent borders just for my solo meanders. However, having taken part in a few AUK events that have started in central or near-central London, including one of the Islington CC rides, I can't say that I found the experience of riding the first and last 10 miles commuter-style to be a great deal of fun. I can do that in the week.
With a bit of tweaking, the Great Escape could be ridden from somewhere such as Epping - accessible by tube - or Brentwood - accessible by train. Of course, that may defeat the object and the idea of getting out of the city by bike, but I think it would make for a more enjoyable day out.