We'll have to re-schedule for later in the summer - how does late August sound?
Sounds good to me.
I don't actually recall any obviously heavy rain around that time in Croydon (but I was going to bed, so may not have noticed), so it could have been quite localised, but if you've gotten that wet, you probably don't want to start a 100 mile night ride, since you'll very rapidly get cold, and possibly get sore patches from rubbing wet material.
That's a bit of a downer for you guys, but it could work out well for me, I should be cycling again in a couple of months, although possibly not at full health and speed.