1. The Tasmin Trail is a nice 12k loop around Richmond Park. It’s offroad tracks are a great way to stay away from the bustle of London’s roads.
https://www.sustrans.org.uk/ncn/map/route/tamsin-trail-richmond-park

2. Greenwich to Gravesend (43k) is another route which keeps in most parts off the main roads as most of the route is on the Thames path.
3. Stepping it up a bit Westminster Bridge to Box Hill is a good route if you want to push yourself. It’s 42k and takes in part of the Olympic cycling route from 2012 so isn’t for slackers like me to try!
https://www.strava.com/local/uk/london/cycling/routes/120
4. London to Brighton is another epic ride which will push even the most accomplished rider. But you can always treat yourself to an ice cream on the seafront to reward your hard work and get the train home!
https://www.strava.com/local/uk/london/cycling/routes/116
5. A stone’s throw from Stratford the Lee Vally Velo Park has a dedicated and floodlit 1-mile road loop which is perfect if you’re looking for the smoothest uninterrupted tarmac in London. It also has many organised courses for beginner and intermediate riders looking to progress their skills further or learn the skills needed for more competitive riding.
https://www.visitleevalley.org.uk/en/content/cms/london2012/velo-park/road/