Hmmm, difficult one to actually enforce apparently is the main issue.....
For what it's worth I would never get on a bike without one now and I think it is the one thing that would send me off the deep end with offspring....
Cycling through Notting Hill some years ago I saw a woman with a very nice hair do come off and crack her head on the pavement. Her two kids who were all helmeted up were terribly, terriblly distressed as Mummy would not wake up and they were there, on that main road, with lots of strangers calling ambulances and generally it all went horribly, horribly wrong for those poor little people. I don't know the outcome but it was a slow motion fall, she just lost her balance, wobbled, whatever, ironically I think trying to keep an eye out for her kids. I suspect, from what I write below, she'd at least have been conscious had she been wearing a helmet.
Two years ago riding home I turned a corner I've ridden hundreds of times to head up a hill. I hit gravel and then the deck, very hard, helmet was a total write off and although I was a bit bruised and had some interesting road rash I was fine. I was doing probably 8 - 10 mph, if that. I'd probably have been in hospital without the helmet looking at the state of it. Interestingly re the above, er, it was the side of my head that was saved.....
Finally I used to teach a lovely girl at riding for the disabled. She was very beautiful and very disabled. She fell of her bike outside her house just before her GCSE year at school while playing around with her friends. She would have got up and walked away had she been wearing a helmet, as it was her totally able and together mind was trapped in a body that didn't work any more. It was very sad to see and even harder for her.
Regardless of what the law is I think it's a no brainer to wear one. Apologies if this seems a rather impassioned post but I've just had so much contact (it doesn't end above) with relatively innocuous situations where helmets would have and have made all the difference.