Like Google (mentioned in another comment), Facebook and Amazon also have large campuses in London with similar headcount, as well as large offices in cities like Dublin.

Google's London office hosts 4000 employees and has cost 1 billion (800m million was the plot). It might not look like a campus, that's because central London simply doesn't have the space. https://www.theverge.com/2017/6/1/15723642/google-london-off...