Well to be honest, you did say 40-50 miles, and with a couple of exceptions, they're all pretty much within that window. I'd avoid Southend for submarine sailing though, because the pond is salt water- I found that out the hard way a few years ago.
There are others, like Hanwell and District, North London Society of Model Engineers (their pond is at Colney Heath in Herts). Lots and lots of clubs to choose from. Nothing stopping anyone from forming one either if there is a suitable body of water and sufficient enthusiasm amongst a small group of like minded individuals.