Good question, well phrased Ray.
With EU legislation being all encompassing on such matters, hey they pretty much set the world standards on helmets and protective clothing, I would have thought there would be few exemptions or exceptions from helmet law.
However, if you visit one or two of the out of the way Greek Islands then you may see some riding without and some of the old geezers on European pedal mopeds in backwater France, Italy and Spain are oblivious to such things.
It was the case and may still be that if you are a Sikh in the UK you are exempt if wearing your turban. Although there aren't many Sikhs wearing turbans on motorcycles. I think I'd have to travel at least 50 miles to see a Sikh let alone one on a motorbike.
Upt'North.