How close do they need to be? Close enough for you to hit them obviously
Although in jest, I make an important point. They can be moving towards you in a threatening manner and they can move a significant distance in a second. So their distance is not much of an issue to when you decide you will have to use force. However, if you make a move towards them, its going to look a lot worse than if you stand your ground, dodge to the side or step backwards whilst delivering the strike.
The second point to make is that whilst the law requires that you show an unwillingness to fight, you have no obligation to withdraw or back down, this used to be the case, but it is no longer. The law now accepts that a person obeying the law should not have to allow themselves to have their lawful business curtailed by threat of violence. Most of us are in professions where it is our job to keep the peace in some form or another so we do have some obligation to stand up to thuggish behaviour.
People keep talking about using minimal force. In order to be able to judge the bare minimum of force you can use, you need to be able to make a precise judgement of the level of force needed and that is just not possible. Section 76 (7) of the Criminal Justice and Immigration Act 2008 outlines that it is not possible to accurately judge the amount of force needed.
“that a person acting for a legitimate purpose may not be able to weigh to a nicety the exact measure of any necessary action; and that evidence of a person's having only done what the person honestly and instinctively thought was necessary for a legitimate purpose constitutes strong evidence that only reasonable action was taken by that person for that purpose.”
The law says "reasonable force" for a reason, its just not possible to judge minimal force.
However, Wingnut makes a very important point. Its not just what actually happens, which is important but other's perception of what happened and what you can show in court. Bear in mind that in court they have to prove you were wrong. It may not be possible to prove every aspect of what happened some of it will rely on who the court believes. But it is useful to be trained in tactics less likely to get you in trouble.
When thinking about what techniques you might use, it is far better to use throws and locks than strikes. Better to use strikes to the legs or arms than the head or body, better to strike the body than the head. Better to use a flat hand or elbow than a punch.
It may also be useful to use verbal commands directed at the offender but to explain to the public what you're doing. Saying in a loud voice "stop struggling you're hurting yourself" is a good counter to them screaming that you're hurting them when you have them in a lock. If someone is being aggressive, holding your hands up (head height) with palms open shows that you are not aggressive and puts your hands in a good place to defend yourself or strike out.
You will (hopefully) defend yourself instinctively. The subconscious takes in far more information, quicker than the conscious. So when you come to write your statement later you won't be fully aware of all the reasons you did what you did. You need to replay it step by step in your head and think about what was said, how it was said (swearing, raised voices, sharp stressed words rather than relaxed and flowing), body language etc. and put that into any statement.
There's a new book out on Kindle by Mark Dawes called Understanding Reasonable Force. It explains all the legal ins and outs of using force and is worth a read for anyone in the security industry.