The case of Wood -v- DPP (2008) makes interesting reading if you want to look at the question of restraint and assault.
I remember many years ago hearing a conversation in which a local hard case boasted that he would sometimes cross the road in order to hit people. It was quite correctly pointed out to him that it was only a matter of time before he picked on the wrong person, ie someone who would turn round and hit him back a lot harder. The 'hard case' was remarkably quiet after this. Perhaps many people already realize the risks involved and avoid physical confrontation for this very reason.
The more I see of life, the more I think violence is best avoided unless there's absolutely no other option. This is because there are plenty of nutters around these days and you never know for sure who they are, or what they're going to do.
Some people will fall on the floor after one punch, and die, while others will refuse to stay down. And it's hard to tell which is which, until it's too late. Look that happened when Ian Tomlinson was pushed onto the floor by PC Simon Harwood
There was a drunken fight in Chichester in the 1980s one lunchtime. The police arrived, and one participant stepped back, while the other completely flipped, and fought the police officers off. At first, there were just two officers and they just couldn't get near him. It was like watching one of those cartoons where everything's a blur, and you can't tell who's who. Then a couple of plain clothes officers arrived to try and help. This wasn't enough either, and after a few more minutes, the area car arrived, because a traffic warden had called for assistance. There were now six officers but it still took them another minute to get some handcuffs on the guy and put him in the back of the police car. The point about this is that at first, it just looked like two middle aged blokes drunkenly pushing each other about, and you really can't tell what someone is like, or what they're capable of. This is probably why so many people don't want to risk getting involved nowadays.
There is no specific definition of reasonable force anyway, and each case is decided on it's own merits. To put this into perspective, society seems to be getting more tense and there's too much violence happening already without adding to it.
I briefly worked in retail about 20 years ago, and a store manager one day asked me to keep an eye on some people who he said were down from London, nicking things. I said if he wanted a security guard, then fair enough, get one, but it wasn't me. Turns out they stole some stereo speakers, but I wasn't about to get stabbed for trying to stop them doing it. I've broken up a few fights over the years, but I still question whether it was worth it. You never get thanked for it, and as with most forms of physical violence, there's always the risk that you could end up getting charged with an offence yourself. Another thing is that it's often the wrong person who ends up getting hurt.
I think what's needed is for more people to take the non confrontational approach. After all, there have been cases of shop assistants and others in public facing roles, getting injured trying to protect revenue, money and items of property.
Isn't health and human life worth more than the cash float in a till, some tat from a shop, or the price of a train ticket ?