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.
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