It is the Queen's Guards who stand guard at the monarch's residences, including Buckingham Palace and, caddy-corner across The Mall, at St. James Palace. At St. James, you can walk up to them (at Buckingham, they're behind a high fence), so that's where tourists mill about to take awkward and suggestive pictures with the guards, who are supposed to stay at attention and not react.
Except, they don't. I was surprised at how young the guards looked. They seemed as if they were barely out of middle school — and they did a pretty poor job of standing at attention. I thought of the Changing of the Guard ceremony I'd seen years earlier, at the Tomb of the Unknown Soldier, in Washington, and how crisp and orderly those guards were in comparison.
Of course, there wasn't a war on then.
With so many soldiers, both British and American, in Iraq and Afghanistan, there probably aren't too many left over for fluff positions like those. In Washington today, maybe the soldiers guarding their unknown comrade are just as nervous and awkward as their colleagues at St. James.