How did I miss this? There's some quality willy waving on this thread.
For the person who said the soldiers are just ceremonial, there have been a number of instances where troops on parade have broken to provide assistance with intruders. On one video shown by blank canvas, you'll see a guardsman stepping in towards the intruder. Certainly down the mall will be additional servicemen lining the route for exactly these reasons and they are instructed as such. There's a significant briefing for these occasions.
As to the 'if they did it to me I'd put him on the floor comments', I doubt you would, or if you did then there are protocols and procedures in place for this also. There are a number of his colleagues behind him who at which point would stop and detain. Whilst not loaded the rifles will have bayonets unsheathed.
Ceremonial duties are exactly that, ceremonial, for which you are drilled, for hours to be in perfect step, time, to respond to the drill sergeants commands.
The only person permitted to deviate the troops or say anything would have been the drill sergeant. That training in ingrained, drilled in over hours and hours of work to get that perfect timing. It's this that someone who hasn't been in the forces probably doesn't understand, which is why I'm trying to explain.
The soldier couldn't have halted. Each move of a squad is controlled with a prewarning and then the move command. A halt by the soldier would have everyone behind piling in like a comedy moment. In an ideal world a swerve may have been appropriate, but the person marching close behind is generally unsighted (especially with bearskins). As such the soldier took a reasonable response in the only way possible to move the person out of the way of oncoming troops by I guess he thought glancing a blow whilst maintaining the required dignified marching pose. Only trouble is it turned into more of a barge.
Bearing in mind he had a few seconds to decide, not the benefit of sitting at a keyboard, pouring over pedantry. I'm sure that decision was taken as it was a man. A woman, OAP, child may have solicited a swerve as an option.
IMHO, having performed ceremonial duties.