The 'Changing the Guard' ceremony at Horse Guards is quite similar to the one that takes place at Buckingham Palace, but it doesn't have any marching bands or music (unless you include the guy blasting out a few notes on his bugle!). But it is also a lot-less crowded, and unlike the ceremony at Buckingham Palace this one takes place every day of the year -- and in all weathers. So you might want to check the schedule for Buckingham Palace first -- if that one isn't running on the day you're going, then this one makes a decent alternative.
The ceremony starts at 10.28 AM, when the replacement guards start riding towards Horse Guards from their home in Hyde Park Barracks. If you've got plenty of energy (and you will definitely need a lot of energy to keep up with the horses!) then you can watch them pass Apsley House and Wellington Arch before riding all the way down Constitution Hill to Buckingham Palace. They will then ride down the Mall and assemble on the parade ground.
A series of changeovers will then take place on the parade ground, with horses riding to and from the barracks in the courtyard. They will also change the horses that are standing in the horseboxes on Whitehall. Unfortunately it is impossible to see both locations at once, so what you have to do is pick a spot beforehand and accept that you will miss half of the show. Luckily Craig has been twice, and seen the whole thing from both locations, so read his review of the ceremony beforehand, because it goes into detail about the timings, and the best place to stand to see as much as possible. He's also included a lot more photographs. Feel free to ask him a question about the ceremony, or write your own review on the forum.
Even if you can't make the ceremony itself, then it's still definitely worth visiting Horse Guards just to see the two mounted sentries standing in the horse boxes (between 10 AM and 4 PM). This is one of the most popular photos for tourists. The guards are not allowed to smile or interact with the crowd, so you will frequently see some students trying to make them laugh. Craig has written a big review about Horse Guards as well.
At 4 PM the horses are replaced by Foot Guards during the Dismounting Ceremony (also known as the 4 O'Clock Parade).
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Admin – “here you go — londondrum.com/transport/trains-journey ... ardsparadeit's a 5 min walk from Westminster stationor you could just get the Piccadilly line all the way to Leicester square and walk from there, past Trafalgar Square and… more”
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