The remaining assassins are grouped on a building with a clock.
What is that building?
It's Shell Mex House [Google Map/Earth Link]
Shell Mex House is a grade II listed building1 situated at number 80, Strand, London, UK. The current building was built in 1930–31 on the site of the Hotel Cecil and stands behind the original facade of the Hotel and between the Adelphi and the Savoy Hotel. Broadly Art Deco in style, it was designed by Ernest Joseph of the architectural firm of Messrs Joseph.
(By Arild Vågen - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=31459889)
Secondarily: the clock reads 4:44, and it's dark. That means it's December/January and thus the Templar crowd spilling out of their HQ are dressed wrong.
I assumed it was 4:44 in the morning so it would be dark. The event starts in the evening so it would seem logical that the end is at night/early morning. I confess though that the interior lighting is a little confused.
If you examine the final scene you reference, there are only a few lights on. If it was 4:44 in the afternoon and dark I can assure you there would be many more office lights on.
I'm not sure what you mean by dressed "wrong"...they all seem to be wearing overcoats or at least clothing suitable for a UK night.
As for 4:44 I can't find any specific special reference to that number being relevant in any particular way to the Assassin's Creed milieu/franchise but I'm not well versed in it.