I confronted a workplace bully and told him to f**k off. In the following discussion he said I had broken some sort of barrier and that he would be justified in hitting me. He threatened to do so. I offered no resistance but said that would probably result in him being sacked. He left the room.

Saying he'd be justified in hitting you isn't hitting you, nor is saying he's going to hit you, and he's the one who walked out.
Of course you can report anything you want, but it could result in both of you being let go. Or one of you. Or being considered someone who can't handle the workplace yourself, like an adult.
You were capable of telling him off, so you are capable of dealing with him in the future.
Next time he bullies, think of something either more clever, biting, or constructive to say than the usual f**k you.
Something like 'Do you have any idea how many people here don't like you because all you do is bully?'
If he's intolerable, find those other people and confront him as a group with one calm person doing the talking.

So after you reported this to your boss and HR what happened?

Sometimes the only thing you can do is tell a bully to f--- off. That might create some more tension but this person knowing that you will stand up for yourself maybe will back down in the future. Threatened to hit but did not, guess what you also used smart words. If this person ever did punch or hit you in the work place. Guess what it would be their a-- fired and gone I would hope. Do not retaliate. This person walked out of the room. Now leave it alone. My suggestion to you is keep track of these incidents. Write everything down and have it ready for yourself. Keep record.

There is a meeting with our managing director planned for tomorrow morning. I'll update later.
Thanks for all the input by the way.

You did the right thing by letting HR know... most of the time these 'bullies' are all talk