The British Royal Family (イギリス王室 Igirisu Ōshitsu) or the Royal Family Faction (王室派 Ōshitsu-ha) is one of the many Magic Side factions of the Toaru Majutsu no Index series. It's the Royal Family of that is comprised of all the relatives of the current acting monarch and are the leaders and rulers of the United Kingdom. They appear as the titular protagonist faction of the the British Royal Family Arc where they manage to prevent Carissa from seizing power.
The head of the royal family is the current acting monarch of the United Kingdom. The monarch is also the Supreme Head of the English Puritan Church and thus, the church answers directly to the ruling monarch. To become a member of the royal family, one must be either related to one of the family members or deomstrate the necessary political skills to become an advisor to the British Royal Family. The famiy are also the primary representatives and symbols of the United Kingdom despite having fewer members than the Knights of England or the Anglican Church.
The family's main headquarters and residence is Buckingham Palace in London with Windsor Castle as their secondary residence as well. Buckingham Palace is where the royal family meets with world leaders and other leaders of the country as well as the members of other British Factions.
The Royal Family consists of mainly the Queen and her committee and is usually supported by two other factions; the Knights of England and the Anglican Church.