Where do I start?

My fastest commute time across London is 45min for 20k, that's 26kph or 16.5mph.
If you can beat that in town at 7.30 you have an easy commute or are suicidal.

Maybe the NorthSouth traffic is lighter than W-E or you don't have 207 traffic lights in your way?

I doubt I average as high as that, but I know from my speedo that I can pass 30kmh on a long straight bit (which admittedly only ever happens if I cycle to the Wharf).