As soon as he grabbed him on the back of the neck or pushed him does this
give you the right to defend ????
Case law provides a precedent that you can actually use a pre-emptive strike.
If you honestly and genuinely believe someone is about to hit you then it is perfectly lawful to give them a smack in the gob to dissuade them from doing so.
As always you have to ask yourself what is reasonable force. If someone is pushing you then yes you can use force to defend yourself.