100 % do wear them or do not ?In the club I helped found and run I'd say it's close to 100% for both men and women. We have no rules or advice about helmets. Neither are we a sporting club.
Oxfordshire I would say is relatively low in helmet wearing overall and that I think reflects the diverse cycling population and high utility usage. I also think there is a slight decline in helmet wearing overall as I think maybe the 'sportive' bubble is bursting and people are finding other kinds of cycling exist.
If I go on a 'Friday's' ride or a Yacf ride the percentage of helmet wearers drops significantly.
Its a tough one to gauge Im up in Lancs at the mo' and it seems to be a bit of hotbed for wearers compared to where I live. I could be wrong it seem the nearer I move to centres of population the more the take up seems to be the one exception seems to be the SE.