As an aside I'm probably in some dire need of tuition regarding the legal ramifications of using MMA in self defence. i have a whole set of options across the force continuum but I don't have a great understanding of which ones I can get away with and when. I can think of situations where I'd likely choke a person out but half if them would probably leave me in jail for use of deadly force.
There is no rule. It is covered by common law. Force has to be 'reasonable' as would be so deemed by a passenger on the Clapham omnibus - i.e. a jury. If you have a fight and the police arrive they will probably arrest you for prompt and effective investigation and after talking to you, to witnesses, and consulting with the CPS form an opinion as to whether a jury or magistrate would convict. If likely they would charge, if not, they would release. A record of your arrest would still show on a DBS check and might prejudice certain employment prospects. Reasonableness of force would also depend on whether you had a reasonable fear for your safety or that of another, proportionality and intent - Defence is licit but not retribution. But even deadly force is legal if it is reasonable.