It depends on how far you want to ride.
I live over by Sloane Square and typically ride south/south west.
I lived in Holland Park for a short while, and I'd ride to Hammersmith, cross the Hammersmith Bridge (closed to cars now but open for cyclists and peds) and head to Richmond Park. You can just do loops in the park or go out from there. A short ride is along the Thames where you can cross at Walton or Bridge Street. You can go farther out to Surrey if you want hills. And a lot of people ride on the river out to Windsor on the Weekends.
You can check out a few clubs, London Dynamo might work for you. Assos Regent St Store runs a social ride on Sundays usually once a month (they post on their instagram feed(
https://www.instagram.com/assos_ldn/?hl=en ). About 80 miles +- and the Piccadilly Rapha Store also runs out a ride.
https://ridewithgps.com/routes/26061890
https://ridewithgps.com/routes/30658931
https://ridewithgps.com/routes/27711481
https://ridewithgps.com/routes/27889388
(last two have some ride photos attached)