The roads in question are very chi chi residential roads, with speed humps, and not really what you would call 'busy' in the context of London (The Evening Standard included a map of their route - I know the roads).
If I had allowed my children to do this (which I wouldn't at 5, maybe at 8), I certainly wouldn't now as all the world, good and bad, knows about them now.
The school has said they will involve SS: doubtless their Child Protection Policy states that they will not release a child into a situation they consider unsafe. Many many schools demand that children lower than Yr4 are met and collected by an adult. What do you think will happen when the school report this to SS?
Nothing - that's what! Because S London social workers have a lot more on their plate than loved cared for children who have been carefully coached in bike safety being out on bikes!