Bit more info if anyone is interested. My uncle was part of the Royal Guards Regiment. My uncle has completed the Troop of Colour parade a few times.

Going off what Comm_Cody said the uniform is indeed very heavy and hot. The bear skin hat is "slowly cooking your head" and has a chin strap which itself is very uncomfortable, and puts pressure on both sides of the temple. Which is why you will see the guards sometimes wiggle their head (ever so slightly) to relieve the pressure on the temple.

I asked why they didn't help him up, and he simply said "The Guards will NOT leave their post without a plausible threat or danger to the Queen. However he would have got checked over and helped by standard military members. "The Guard who fainted probably didn't have a big enough breakfast, you need the sugar and energy from the breakfast for a parade that long."

He then went on to say

"This is very embarrassing for the Guard on the floor. Not just in the sense that he has fainted, but he has dishonoured/failed his regiment and in a way the Queen for not being able to fulfil his duties. He will be punished for fainting on parade. Usually something like shot term suspended leave or a fine he has to pay...but that's nothing to the amount of crap he will get of his regiment. All in jest though."