Lost Taiwan: MI6

The St. Ermin Hotel is a late Victorian building that was converted into a hotel in 1896-99. During the First World War, it became a meeting place and headquarters for British Secret intelligence services, also known as the MI6.
On January 16, 1917, as a German spy in the UK. You received the intel from your commander that the British army has possession of the Military decoder. So you sneak into the MI6 headquarter to steal some important information and code. In the process, you’ve got intel about a secret area of MI6 and after comparing it with the official building plan, you found a hidden passage. You confirm that the information you want to obtain is in this area. Can you complete the mission and change the history of World War I?