Of course, you don’t have to wait to get thumped before you throw a punch in self defence. However the continued attack whilst the bloke is on the floor is not self defence-unless there is something we cannot see or comes from witness accounts-inc possession of a weapon/threat of or perhaps knowledge of some fighting skills-all v unlikely but self defence is a subjective honestly held belief not a third person, objective test.so you are an MP and threatened in the street - or in your constituency office - are you suggesting that you take the first knife blow before you throw a punch? You know - to make it fair - make sure the nutter plays his/her hand before you defend yourself?
See for example two jags.