I am a big fan of Boris Johnson, and I read this with much delight.
I was once rabidly pro-helmet, but I now think that if you don't mind splitting your skull open on the pavement, then I shouldn't try and stop you.
I prefer to sit in the "collision avoidance" camp, rather than the "be protected after a collision" camp. That means wearing bright, reflective clothing; having a well maintained bike with good brakes and tyres; keeping a lookout (and not stuffing earphones in your listening devices) and having a really powerful light front and rear for night time riding.
I have seen plenty of idiots riding badly maintained bikes in dark clothing with no lights in the pitch dark - but still wearing a helmet. A helmet should be the last safety device that one should acquire.