Surprisingly this isn’t true. The marathon was due to take place in April when the average high temperature in London is 15 degrees and the average low is 7 degrees. It will now take place in October when the average high in London is 16 degrees and the average low is 10 degrees. The only real difference between the two months is the amount of daylight, but this won’t really impact most participants.
I work for a charity with lots of people running to raise funds for us and this will have a big impact. Hopefully, it is only a postponement, not a cancellation.