It probably isn't fake as dressing up in the uniform and carrying a replica gun doesn't go unnoticed for very long in London. The Mall is in the background with no traffic so it was shot on a Sunday, probably in the afternoon.
The shot has been taken into an edit suite and the graphic inserted over the slomo so it has been handled by professionals. We have to wonder at the timing of the release of the video, if not obvious now it may become apparent what the Sun are up to (if anything) in due course.
The idiot mimic may well be speaking during the entire walk, so it may be something he said that triggered the response. (or the tone of what he was saying if the guard did not understand the language)
It is curious as to why the clip apparently ends where it does. The reaction to stop filming seems too fast for an amateur who would tend to take a few seconds longer to react. Looks like the end of the clip has been edited for maximum effect, which suggests that the next few seconds after the guard pushes the idiot are less dramatic, like maybe the guard quickly gets back to marching and the idiot makes an apologetic gesture.
If it is a setup then it is VERY clever has they didn't go overboard with an OTT reaction.
Would like a translation of what the idiot is saying
Mickjoebill