I thought this might be an interesting topic.
I was out shopping with the kids today and it was straight after school in the supermarket. My boss rang me on the mobile and I had to answer it as a new job is coming up. The kids acted up cause they had been in school all day. I told them off severely. They were waiting past the checkouts, one of them was trying to post a receipt into a receipt printer and I did the stern "no" dad bit. This bloke came up to them and he was about 60 + but not infirm and he said you are being bad and I thought he was going to do the distracting grandpa act, you know there's a pigeon out the window can you see it sort of thing, but then my kids turned away from him (cos I teach them not to talk to strangers as you do). He said to me 'You're creating a rod for your own back, its too late they should be put down". I was about to give him a few choice F words when I thought no I'll be civilised and show the kids a good example. I said "How dare you talk to my kids like that! Go away, go away now!" obviously in a shouty angry fierce way. He went quickly off passing comment over his shoulder.
Anyway this led me to think what would be reasonable to have done if he'd laid hands on the kids and I'd have lost my temper. So I looked up somethings, apparently an assault doesn't have to be physical, it can be implied, so if I'd have said go away or I'll knock the crap out of you that could have been construed as assault, but if he'd touched or I thought he was going to touch the kids I would have been defending them and a threat could have been deemed as reasonable to get him to go away.
Check out this website if you want to try and get your head round it. Maybe someone else knows a bit more about it if they are lawers or in the police.
The law on assault