No idea about Bristol.
I met an Aussie guy a couple of years back who worked in traffic safety. Their studies said that most kids brains can’t process all the info required to deal with riding on the roads until they are ~13. Make of that what you will.
My personal experience with my lad now 10, is that he has only just got the hang of riding on the road with due regard for himself and everyone else, and he’s been riding regularly on the road since he was 7.
Being able to interpret what is happening, and what might happen though we take it for granted is pretty tricky for kids.
I reckon they need experience, and that comes with time on the bike on quieter roads.