Just throwing it out there but if you wanted to go south of the river:
Brixton -> Clapham -> Wandsworth and then along the Upper Richmond Road until you hit Mortlake and then cross the river at the tip of Kew Gardens.
I always enjoy being by Clapham Common for a little bit and the upper richmond road is mostly a nice high speed blast with few traffic lights and a decent amount of space so traffic doesn't always slow you down.
Just throwing it out there but if you wanted to go south of the river:
Brixton -> Clapham -> Wandsworth and then along the Upper Richmond Road until you hit Mortlake and then cross the river at the tip of Kew Gardens.
I always enjoy being by Clapham Common for a little bit and the upper richmond road is mostly a nice high speed blast with few traffic lights and a decent amount of space so traffic doesn't always slow you down.
12.5 Miles apparently so adds a few to your journey..