Out of the saddle? I don't know anything steep enough to get me out of the saddle. Don't you have gears?A bit closer to central London than Croydon, there's Primrose Hill, which is steep enough to get you out of the saddle, but pretty short. Probably just about doable on a Boris Bike if you fancy a challenge.
The climb up to the overpass from the High Street in Croydon can be challenging if you've just ridden to Brighton and back.Water Tower Hill, which does run North South is a bit steep.
Canonbie road.
Someone at the back with possibly a gearless bike walking after straying from hipster flatlands
https://m.flickr.com/#/photos/37034456@N06/4720544170/