Trafalgar Square is the public square around Nelson's Column, with the National Gallery and St Martin-in-the-Fields on its edges and Charing Cross station beside it.
The square is built around the Charing Cross junction, so Charing Cross station on the Strand side sits right next to it. Ubisoft's concept art for Syndicate includes a Trafalgar Square piece. The square is also seen in the Rifts of Assassin's Creed: Brotherhood, though Syndicate is its first full appearance.
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.
Adelphi Theatre
The Adelphi Theatre is a London playhouse in the Jack the Ripper expansion. It ties into the Woody Shinnings bounty, where Inspector Abberline asks Evie to bring in the understudy who murdered the actor Richard Altman.
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.
Alhambra Music Hall
The Alhambra Music Hall is Maxwell Roth's theatre on Leicester Square in the Strand, the Blighter leader's base and the stage for Jacob's final showdown with him. Several of Jacob's memories with Roth start here.