It's worth noting that their job has zero room for errors. They are expected to be basically invisible, outside of the ceremonial parts. They are also (I believe) authorised for live fire, at their own discretion.

They walk a political tightrope, and the last major fuck up I heard about was decades back now.

Kings Guard is just an assignment given to several regiments of the British Army. They would be doing other duties, training and deployments otherwise.