MetPolice: Diplomatic Protection Group(D.P.G.)

The Royalty and Diplomatic Protection Department (RDPD) is a branch of the London Metropolitan Police. It currently contains three separate sub-departments: The Royalty Protection Group (SO14), the Diplomatic Protection Group (SO16) and the Palace of Westminster Division (SO17).
The security, as distinct from the ceremonial bodyguards or military protection, of the Sovereign of the United Kingdom and of members of the British Royal Family is entrusted to the Metropolitan Police. As currently organised, the Assistant Commissioner, Specialist Operations, has overall control. He exercises this through the Deputy Assistant Commissioner, Security and Protection. This DAC controls the Royalty and Diplomatic Protection Department, Aviation Security (SO18), and Special Branch (SO12). The Royalty and Diplomatic Protection Department has primary responsibility for the safety of the Sovereign and the Royal Family and of visiting overseas VIPs and their accredited representatives in the United Kingdom.

http://www.met.police.uk/co/protection.htm

http://www.ukemergency.co.uk/police/dpg1.htm