St Margaret's is the parish church in the grounds of Westminster Abbey, the church of the House of Commons, and a database landmark in Syndicate.
Its crowded churchyard became so foul by the 19th century that doctors blamed it for bad air in Parliament, so the graves were moved.
The church stands within the Westminster Abbey grounds, across from the Palace of Westminster, so you can collect its database entry on the same trip as the Abbey and Parliament.
10 Downing Street
10 Downing Street is the Prime Minister's home and office in Westminster. In 1868 Benjamin Disraeli lives here, and Jacob's work as his bodyguard in Sequence 7 starts outside the house.
Admiralty
The Admiralty is a government office on Whitehall in Westminster, one of the period buildings you can climb and read about in the in-game database. No memory centres on it.
Banqueting Hall
The Banqueting Hall, later the Chapel Royal, is a 1622 hall on Whitehall in Westminster and a database landmark in Syndicate. It is the only surviving part of the old Palace of Whitehall.
Blue Anchor Alley
Blue Anchor Alley is a Blighter stronghold in Westminster run under Lilla Graves, one of two in that borough. Freeing it lets the Rooks set up there instead.