Sorry, I should have said that I agree that the Sec 20 offence is more the likely outcome, IF you admit or are found guilty. Self-defence is a statutory defence to assault and The burden of proof remains with the prosecution when the issue of self-defence is raised.
The basic principles of self-defence are set out in (Palmer v R, [1971] AC 814); approved in R v McInnes, 55 Cr App R 551:
"It is both good law and good sense that a man who is attacked may defend himself. It is both good law and good sense that he may do, but only do, what is reasonably necessary."