The Law states that if an attacker is aggresive and moving forward, and you fear for your safety, you can legally Pre-emptively strike the first blow in Self Defence.
If you are put in fear for your safety by his aggresively advancing into your space he is committing Common Assault.
He has breached the Law and this entitles you to launch a PE.
The Criminal law act (1967) states:
you must restrict yourself to using "only such force as is reasonable in the circumstances".
The Police and Criminal Evidence Act (1984) states:
you must use "the minimum force necessary to achieve this objective".
Landing one PE and stepping away keeps you within the law.
(Kicking him when he is down or pulling out a weapon does not!)
THAT is how we use a PE legally in a Self Defence situation.