There's the Sunday London Ride that meets at Hyde Park Corner at 9.30 every Sunday. I've not been on one for a while. Other than that just keep an eye on the Informal Rides and see what comes up, there's usual at least one hundred miler a month on here as people are doing the Century a Month challenge (so was I but it looks like I'll fail in January).
There's a ride to Warwick coming up in May and I'll certainly be looking to organise something in Feb.