The men you can see in front of Buckingham Palace are the Queen’s guard. They are mostly Foot Guards (The Grenadier Guards, The Coldstream Guards, The Scots Guards, The Irish Guards, The Welsh Guards).
You can also see them at other royal palaces together with Household Cavalry.
You can make a difference among the royal regiment uniforms by this picture.
From May to July the Changing of the Guard takes place every day at 11:30 a.m. in front of Buckingham Palace. At other times of the year the ceremony takes place on alternate days.
Due to weather conditions or other reasons, the ceremony can be cancelled. It is Captain of the Guard responsibel for that decision and it can be made right before the ceremony officially starts (10.45 a.m.)