Cycling Fitness / Training Advice

I can’t see a 20 minute hill in the South East that I could do a test outdoors on.

Whereabouts in London are you? I can do a 20 minute test in two places in south-east London;

1. The A21 Gates Green roundabout to the Shampan curry house in Biggin Hill https://www.strava.com/segments/2348729

2. Corkscrew Hill roundabout to the B265 - can't find the exact Strava segment but continue this one to the junction with the B265 https://www.strava.com/segments/12841629

They are not perfect, as both have a couple of shorter downhills, but you can whack it into a big gear and keep the power on.