Fainting on parades is a very common thing and not a phenomenon that has appeared after COVID. Just look at any pre-pandemic article about trooping the colour and there will be at least one person fainting.

Doing physical work for 12 hours is totally different to standing completely still for hours on end. Standing still affects blood circulation negatively which is why the Coldstream guards (not a beefeater), regularly march to different guard spots or switch the arm that they hold their rifle with.