Anyway this is the basic idea for legal defence against assault.
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The issue that you will run in to is that self defence generally says something along the lines of justifiable fear or you feared for your life. Or something like that. And so people interpret that as if I am convinced I was afraid for my life then my actions were justifiable. Eg. Soccer kicking someone in the head.
In practical terms though you have to convince the judge or the jury. And they can be a bit weird on the subject.
Hence why you would look at case law or precedent rather than statute law.(if I have my terms correct, which I probably don't)
I have never encountered a judge who has looked kindly on kicking people while prone exept in exceptional circumstances.
And I have never heard but there were no witnesses so therefore my story is the correct one to carry much weight in those sorts of instances.