Westminster Abbey is the coronation church west of Parliament in Westminster. The Blighter Harrison Harley plans to burn it down, and his Bounty Hunt has you kidnap him first.
Harley is a minor canon of the Abbey who has turned against it. The twins grab him and hand him to the police before he can set it alight.
Harley has served the Church of England all his life but now sees the Abbey as corrupted and means to burn it. Grab him as Jacob or Evie and walk him to a carriage; he will try to wrench free and call for help, so be ready to catch him again. Deliver him to Sergeant Abberline to finish the job.
The Abbey's database history includes a series tie-in: at Charles II's coronation in 1661, the king spotted his general George Monck talking with a man who held an Apple of Eden, but had to bow for his crown before he could look again.
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.