ENGLAND, LONDON, BUCKINGHAM PALACE, QUEEN'S ROYAL GUARDS & PICCADILLY CIRCUS.

ENGLAND, LONDON, BUCKINGHAM PALACE, QUEEN'S ROYAL GUARDS & PICCADILLY CIRCUS.
Wikipedia
The Queen's Guard, British Guards, and Queen's Life Guard (called King's Guard and King's Life Guard when the reigning monarch is male) are the names given to contingents of infantry and cavalry soldiers charged with guarding the official royal residences in the United Kingdom. The British Army has regiments of both Horse Guards and Foot Guards predating the English Restoration (1660), and since the reign of King Charles II these regiments have been responsible for guarding the Sovereign's palaces. The Guards are fully operational soldiers.
The Queen's Guard is the name given to the contingent of infantry responsible for guarding Buckingham Palace and St James's Palace (including Clarence House) in London. The guard is made up of a company of soldiers from a single regiment, which is split in two, providing a detachment for Buckingham Palace and a detachment for St James's Palace. Because the Sovereign's official residence is still St James's, the guard commander (called the captain of the guard) is based there, as are the regiment's colours. When the Sovereign is in residence, the Queen's Guard numbers three officers and forty other ranks, with four sentries each posted at Buckingham Palace (on the forecourt) and St James's Palace (two in Friary Court, two at the entrance to Clarence House). This reduces to three officers and 31 ORs, with two sentries each, when the Sovereign is not in residence. The Queen's Guard is not purely ceremonial in nature. They provide sentries during the day and night, and during the later hours, they patrol the grounds of the Palace. Until 1959, the sentries at Buckingham Palace were stationed outside the fence. This stopped following an incident involving a female tourist and a Coldstream Guardsman - due to the continued pestering by tourists and sightseers, the guardsman kicked the tourist on the ankle as he marched. The tourist made a complaint to the police and the sentry was confined to barracks for ten days. Not long after, the sentries were moved inside the fence.
Band of The Scots Guards leaving Victoria Barracks
At any time, three infantry battalions are posted for public duties; two of these are Guards battalions (one based at Wellington Barracks next to Buckingham Palace and one at Victoria Barracks in Windsor), while the third is a line infantry unit (based at the Royal Artillery Barracks, Woolwich). In addition, there are three incremental companies based at the Royal Artillery Barracks and Wellington Barracks. All of these units come under the administrative authority of London District - as public duties units, they not only take part in ceremonial but are also committed to providing military aid to the civilian authorities.
Buckingham Palace, St James's Palace, and Tower of London
There are two detachments of the Queen's Guard in London, one each for Buckingham Palace and St James's Palace, under the command of the captain of the Queen's Guard. Because St James's Palace is still the official residence of the court, it is here that the colour is lodged and the captain of the guard establishes his headquarters.
The Queen's Guard in London changes in the forecourt of Buckingham Palace at 11:00 am every day in early summer and four times per week otherwise.
Band of the Welsh Guards march at St James Palace
The St James's Palace detachment of the Queen's Guard, led usually by the corps of drums, and bearing the colour (if the Queen is in residence, then this will be the Queen's colour; if she is not, then it is the regimental colour), marches along the Mall to Buckingham Palace, where the Buckingham Palace detachment has formed up to await their arrival. These two detachments are the old guard. Meanwhile, the new guard is forming up and are awaiting inspection by the adjutant on the parade square at Wellington Barracks. The band, having been inspected by the adjutant, forms a circle to play music whilst the new guard is inspected. The guard provides a full military band consisting of no fewer than 35 musicians (usually, although not always, from one of the Guards regiments) led by their director of music. When the new guard is formed up, led by the band, it marches across into the forecourt of Buckingham Palace. Once there, the New Guard advances towards the old guard in slow time and halts. The old guard presents arms, followed by the new guard presenting arms. The captains of the guards march towards each other for the handing over of the palace keys. The new reliefs are marched to the guardrooms of Buckingham Palace and St James's Palace where new sentries are posted.