Just as I was about to leave this morning there was a downpour so I decided to sit out the worst of it, hanging around the house in my bike gear. Whereupon my 2-year-old turned to me and said "Just go!". Thanks, MicroEthel! As the downpour passed shortly afterwards I followed MicroEthel's advice.
The wind was blowing the rain the same direction as I was going, and by around halfway it felt like I was catching up with it. So I did a loop of the park to give it another head start; that's my excuse and I'm sticking to it!
Allegedly there's a bus strike in London today. Not many buses along my route anyway, but I noticed no difference to traffic levels.