[She-Ra]

It's too blurred an area for me to give exact references, what is generally known about it comes from the self defense section of UK Common Law. Everything I've seen on it before are general reviews because it's not statutory, and seems to be dealt with by individual case with specific legal advice and for those dealing with it to decide at a later date whether the behaviour came under a statute or remains under common law.

Just did a quick brush up and it seems the having to warn the other person(s) is no longer but martial artists have a duty to avoid conflict but if they cannot then like any other person they have to use reasonable action and if the fight escalates, escalate accordingly and if it lessons, then lesson their action too. BUT, in practice, in court they will have to prove that your behaviour was excessive and to determine whether it was self defense or revenge, they will examine that in hindsight obviously. In that situation, people will generally put the onus on the trained person to know better and even though it could have been situation that happened quickly and without warning people will generally expect the practitioner to have better instincts. It can be easily argued in all sorts of ways on an individual basis so the advice is generally to try and retreat or show reluctance and if that doesn't work then to try and neutralize the threat efficiently and without 'too much' damage in which case jujitsu is well designed.

How to define a street fight - exactly. That was just my terminology - but just not doing anything too obvious, that a non-practitioner wouldn't likely do is a safe idea. The public and courts aren't likely to look down on you for blocking, but if you have an obvious advantage and use it too well, people don't tend to like that.