Seems like an extreme reaction. I assume the dude was a tourist.

This immediately came to mind

It's fake.
That doesn't look like a real queens guard/royal marine uniform, no weapon, no verbal warning, he's standing in an open pathway 1/2 blocking people, and the overreaction to what would likely be a daily nuisance.
Also, it's from this "social experiment"....
https://youtu.be/vv007iMWRMY

And there's no way that punch would have kayoed anyone without a serious medical condition.

And, and, and....

Yes, it surprises me anyone thinks this is real.

Come on... when's the last time you were in a brawl? Have seen lots of guys go down with less than that, caused a few of 'em too. A guy back home (Mike Lubbock) was about 6' 1" tall, big broad shoulders and barrel chest, would sooner fight than eat and used to win most of the time. Unless you caught him just below the right cheek bone, and he'd drop like a soft rag. Usually woke up after a minute or two. The Metheral brothers were about the same, and so was Keith Verstagen. Doesn't take a serious medical condition, just one of those things.

The overactive camera movement, trying to capture the reactions and results, is always a give-away on these, as if everything else in this skit didn't give it away. The only thing missing is the paid extra, screaming directly into the camera, "Blimey, call 999!"

It looks fake. The Guradsman does not wear a belt, has no weapon, has no chinstrap on the bearskin, etc. etc.
By the grouping of the buttons on his tunic, he should be of the Coldstream Guard, their full uniform looks like this:
https://changing-guard.com/images/foot-guards/coldstream-sentry.jpg

fake as hell, the ill fitting tailoring gives it away. Beefeaters are anything but shabby

He's not a "Beefeater", either

I know!

https://www.youtube.com/watch?v=vv007iMWRMY