With Sites in RefinedTheme, you can set the landing page for user groups as they log in.
Log in as a user in the London office, the user is shown the global site home. Log out.
Log in as an admin user.
As an admin, go to the Site Builder.
Edit the site you want to be the start site for the london users, and add the user-group to the setting "Start site for groups". Remember to save.
Make sure you have the user groups setup in regular Confluence, and the users added to the user groups.
Log out, then log in as the London Office user again. The London site is shown.
Repeat the steps but for the Paris office. Logging in as the Paris Office user, you will see the Paris Site Home first.