It is partly, but I think police have reasonable reason to be a little concerned.
Look into the Henry Hicks case I posted above, police were charged (although cleared) of gross misconduct and he WAS wearing a helmet. If someone wasn't wearing a helmet it may have been different.
I'm no expert...
Yep, it was a BBC episode on the MET about the Henry Hicks case (who was wearing a helmet), whereby police were charged (but cleared) of gross misconduct which shows why police are far from keen to chase anyone without a helmet on a moped. There's plenty to say that it's an 'unwritten rule', but...