Absolutely, even behaviour that is 'threatening' could warrant 'self-defence'.
There is no statute about 'assault' or what constitutes reasonable self-defence, it is common law. The question which is asked of the jury is
"Was the force used by the individual reasonable in the circumstances as he or she believed them to be?"
Note the last phrase - as a crude analogy if someone swung at you with a hammer but it was made of foam, you'd probably be justified in knocking them out

If a person believes there is a threat of imminent violence to themselves, their immediate family or even a total stranger, they can act in 'self-defence'
Even if a person kills someone, wrongly, if the jury decides they genuinely believed that they were in imminent danger they will be acquitted (if they act proportionately of course)
Also, whilst the option to retreat can be taken into account, it is not mandatory nor does the lack of doing so in itself sink a self-defence defence, if the person is believed to have acted instinctively.
(I am not a lawyer)