We saw these horse-drawn carriages outside the gates of Buckingham Palace. I am guessing that they were from the Royal Mews where the State vehicles are housed. These carriages are used for Royal and State occasions, State Visits, Weddings, etc.
Since 1843 the daily messenger brougham has set out from the Mews to collect and deliver post between Royal households.
To prepare them for London traffic, every day a horse-drawn Brougham carriage sets out to collect and deliver royal packages around London, including the Palace’s weekly copy of "Country Life".
Interestingly, did you know that a bolting horse almost caused a pandemonium during the royal wedding procession to Buckingham Palace?
Here's an excerpt from the Mirror.
"The newlyweds had a narrow escape when a horse bolted in front of their carriage as they left Westminster Abbey – causing panic among the royal procession.
A black horse from the Household Cavalry was spooked by the crowd, causing it to rear and crash to the ground – dismounting its rider.
Although the guard tried to restrain the horse, it was too strong and charged up Whitehall, overtaking William and Kate’s 1902 State Landau carriage.
One onlooker said: “Kate looked shocked and worried but William calmly spoke to her.”
Thankfully, no one was hurt and the horse was herded to its box."
The Horse Guards Parade at the Old Admiralty Building
"The hooves of horses!
Oh! witching and sweet
Is the music earth steals from the iron-shod feet;
No whisper of lover, no trilling of bird,
Can stir me as much as hooves of horses
~Will H. Ogilvi
"Look back at our struggle for freedom,
Trace our present day's strength to it's source;
And you'll find that man's pathway to glory
Is strewn with the bones of the horse."