My experience in traffic is that speed is mostly dictated by traffic lights and congestion, not how fast your bike can go. But having to stop and fix a flat will impact your average speed even more, so for me reliability and puncture protection would take priority over a few watts of rolling resistance.
Of course, I've never commuted in London, only visited, but based on that (and watching Richard trying to beat the Stig) it looks like quite a challenge!