The GWR Paddington Express is a good one, I think that's in April. Bristol and back, essentially, with lots of Brunel along the way.
ACH's White Hart is also April.
There's the London Circuit 400 which is fun if you want to draw a giant circle on Strava, though that clashes with...
Fast Times in the High Weald 200 which is in May on coronation day and is filling up nicely - it's hilly for our part of the world but very scenic... : )
I also DIYed Paragon's Oh! To Be In Kent last year and that's another nice route.