The Queen's role is ceremonial now but they still carry on with the tradition of Changing of the Guard. I would suggest you arrive at Buckingham Palace by 11:15 a.m. so that you can see the 30 minute ceremony. It is held daily from May to July, and on alternate dates throughout the rest of the year.
There are 3 Officers and31 men when the Queen is away and 40 when she is in. You can tell if she is in residence, if the Royal Standard is flown. Also if she is in residence, there are four guards in the front and if she is not there there are two.
There will be no Changing of the Guard in wet weather.
All the Best!
American to Britain