There are many similarities between America's Sentinels of the Tomb of the Unknown and the United Kingdom's Queen's Guard. Both are highly respected positions within their respective armed forces, both remain stoic in the face of terrible weather conditions, and both will readily put disrespectful tourists in their proper place.
The Queen's Guard silently stands watch at the Royal Residences and, throughout the years, have become more ceremonial than practical, as the task of protecting the queen has been given to the Metropolitan Police. Still, they remain outside in case the worst happens.
Of course, this doesn't stop tourists from trying to provoke the motionless sentries. Many tourists try to get a smile out of the guards with silly jokes and faces — there even reports of women flashing them just to get a reaction. The highly trained sentries will bite their tongue at mild distractions. Former sentries joke that this is just part of the position. They can't ever show it, but they like it when tourists take photos and act politely.
It's when the tourists really get in their face — poking them with pins, putting cigarette butts out on their rifles, anything like that — then they can act accordingly. In the case of tourists getting way too handsy in photographs, they'll wait until the last moment to ruin the picture by marching away. If you block their movements, they'll shout, "make way for the Queen's Guard!" If you get in their face or if they have to shout too many times, they'll knock you out then stoically resume their post.
If idiots act threateningly towards the Royal Family, the Queen's Guard, or the general public around them, they will stop you.
If you touch their bear-skin hat, they'll probably ignore you or shout at you. If you grab their rifle, the next thing you'll see is the end of their barrel.
For more information on the Queen's Guard and how they react to disrespectful tourists, watch the video below.