Shortish but fast routes in London

Embankment / Upper thames Street is fucking mental, I wouldn't try to ride there recreationally ever unless in a group.

That was my commute a couple of years back and my very first thought on reading this thread. Best commute ever - fast roads, with lights you can see way in the distance so their timing's clear enough that you can slow down and coast through as they go green or attack hard and fly through on amber. At the end of it there is a short little rise up to Tower Bridge and then you can nip over that and hit south London, or you can head down The Highway for a little respite before another long, clear straight. Depending on how hard you want to bring it, head right and go along the river for a quick rest, or head into the Limehouse Link - not strictly allowed, but doable - and pop out the other side (keep left until you're out the tunnel) and head right into Canary Wharf. Marsh Wall gives you another stretch without any lights, but I prefer to go round the island on Westferry Road before taking a left on to the A13 which becomes Commercial Road and heading back towards civilisation and bagels.