Westminster Abbey (via Wikipedia):
It is one of the most notable religious buildings in the United Kingdom and has been the traditional place of coronation and burial site for English and, later, British monarchs. Between 1540 and 1556 the abbey had the status of a cathedral. Since 1560, however, the building is no longer an abbey nor a cathedral, having instead the status of a Church of England "Royal Peculiar"—a church responsible directly to the sovereign. The building itself is the original abbey church.
According to a tradition first reported by Sulcard in about 1080, a church was founded at the site (then known as Thorn Ey (Thorn Island)) in the 7th century, at the time of Mellitus, a Bishop of London. Construction of the present church began in 1245, on the orders of King Henry III. […]
Coronations
Since the coronations in 1066 of both King Harold and William the Conqueror, coronations of English and British monarchs were held in the abbey. […]
King Edward's Chair (or St Edward's Chair), the throne on which English and British sovereigns have been seated at the moment of coronation, is housed within the abbey and has been used at every coronation since 1308. From 1301 to 1996 (except for a short time in 1950 when it was temporarily stolen by Scottish nationalists), the chair also housed the Stone of Scone upon which the kings of Scots are crowned. […]
Chapter house
The chapter house was built concurrently with the east parts of the abbey under Henry III, between about 1245 and 1253. It was restored by Sir George Gilbert Scott in 1872. The entrance is approached from the east cloister walk and includes a double doorway with a large tympanum above. […]
A door within the vestibule dates from around 1050 and is believed to be the oldest in England. The exterior includes flying buttresses added in the 14th century and a leaded tent-lantern roof on an iron frame designed by Scott. The Chapter house was originally used in the 13th century by Benedictine monks for daily meetings. It later became a meeting place of the King's Great Council and the Commons, predecessors of Parliament.
The Chapter House is a profoundly important site in English history, where democracy and the idea of 'Parliament' took initial, tentative steps, and the absolute authority of the monarchy was eroded.
In 1258, bankrupt and threatened with excommunication, Henry III was forced to agree to the Provisions of Oxford, under which his debts were paid in exchange for reform. A Great Council of 24 assembled, partly nominated by a group of barons, assuming the functions of the King's Great Council.
The agreement also called for Parliament to meet regularly, with representatives from the counties convening here, in the form of the early House of Commons until 1547 before transferring to the Palace of Westminster.
To this day the Chapter House, an enclave of reason and human law in a house of God, remains under the control of Parliament and not the Abbey.
While the Abbey makes every effort to hide the fact, the Chapter House is free to enter. Expect to be challenged, but don't be put off, and definitely don't miss the medieval tiled floor of the Pyx Chamber, which is perhaps the finest in England.