After 3 weeks commuting I now find the bottleneck is truly the traffic itself. Recently I have been cycling in a more relaxed way, mostly on lower gear (except downhill or flat, though London is never really that flat), stop peddling when I see a red light from a distance. I spend about the same time as when I worked really hard, sometimes even earlier depending on the red lights I got. Next year I may try cycling the whole route using my big bike when there is more daylight.