London ceremonial policeman

This is a photo of a ceremonial London policeman outside one of the army regiments. I obviously do not know more than that, I initially thought that this was what they called a London 'bearskin', but a search for that term brought a different kind of uniform, and then I recalled that this photo was actually taken outside a place where the placard mentioned that this was the headquarters for one of the regiments. Of course, if someone is reading this and knows which regiment, please update via the comments so that I can put that information in.

Finally got the correction via comments: (From Martin)

This not a policeman, it is a soldier from the Household Cavalry's Life Guards regiment (Distinguishable from the Blues and Royals by the white plume). He is on duty outside Horse Guards.Horse Guards is named after the troops who have mounted the Queen's Life Guard on this spot since the Restoration of King Charles II in 1660. It remains the official entrance to St James's Palace and Buckingham Palace and this is why The Queen's Life Guard is still mounted here.Apart from members of the Royal Family or cavalrymen on duty, everyone needs the Sovereign's permission in the form of an Ivory Pass to either drive or ride through Horse Guards.