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.
The twins follow a lead from Henry Green into the Blue Anchor Alley stronghold memory. With the Blighters cleared out, it becomes a Rook stronghold and weakens Graves' hold on Westminster.
The leader rallies his men in Lilla Graves' name, mourning fallen Blighters and promising to crush the Rooks. Because full sync asks you to kill him undetected, keep to cover and pick your moment, and look for the heist plans to burn.
Lilla Graves also relies on the Templars Wallace Bone, Beatrice Gribble and Edgar Collicott, on Blighter targets Harrison Harley, Ivan Bunbury and Wade Lynton, and on two child-labour sites, the Red Growler Distillery and Goodfellow's Brewery.
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.
Buckingham Palace
Buckingham Palace is Queen Victoria's London home in Westminster and the setting for the climax of Syndicate's story, the Queen's ball. It is also where the Queen hands out the Operation side memories, and it has a viewpoint.