Sure it's dangerous, but they are doing far more dangerous things on their bikes.
So I think a foam plane is nothing to worry about compared to some rotors spinning...
What they to themselves do is immaterial.
Air Navigation Order Article 74
“A person shall not recklessly or negligently cause or permit an aircraft to endanger any person or property.”
This includes model aircraft.
Foam model travelling at 40-50mph into someone's face - still nothing to worry about?
Consider the consequences of a slight misjudgement or equipment failure where there is no real margin for error. You have already stated yourself that it is dangerous. That's not much of a defence in an insurance claim against you. I don't want to start an argument here so maybe I'll just leave it at that.