From Halopedia, the Halo wiki
At some point prior to 2170, the Palace of Westminster was badly damaged in an act of domestic terrorism. The United Kingdom ceased to exist as a governing entity shortly thereafter (though the term still continued to be used for areas of its former governance). Pieces of the Houses of Parliament, as the palace was also known, were gathered and some were restored. The symbolic action of such a restoration was vital to the creation of the UEG, and was also seen by some as a cynical attempt to play on the twin vices of nostalgia and patriotism.
In the late twenty-fifth century, restored pieces of the original building were fashioned used to create a cavernous space where trials were held.
During the Battle for Earth in the final stages of the Human-Covenant War in late 2552, the Covenant attacked the city with Type-57 Phantoms, Type-26B Banshees, and Jiralhanae patrolling several areas. While UNSC M808 Scorpions were deployed, they were soon left abandoned. Sometime after this, multiple Spartans were deployed into London via drop pods. A team of them successfully rescued a civilian from a group of Jiralhanae, with one of the Spartans being injured in the process.
In January of 2558, a trial for the smart AI known as Iona was held in a simulated environment modeled after this space. Later that year on October 28, Cortana and her Created sent two Guardians to enforce martial law on Earth, subjugating London in the process. London and the rest of Earth would remain under Cortana's control during the Created conflict.
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