Matt Bird writes all sorts of nonsense, but he dedicates a large chunk of his time to writing game walkthroughs.
After hacking your way through a security door, you'll find yourself in the main hall of the Buckingham Palace Underground. Prepper will complain about losing you on the radio, but don't worry—there's little to nothing to fret over down here.
As soon as you enter the main hall, check a fallen piece of furniture opposite the stairs for a Capacity Upgrade. Then turn around and scan the walls near the front doors for a secret message on a painting, only visible via the Prepper Pad. Once that's done, check the hallway that's red-lit for a painted sign of a Raven—a sure sign of a save spot. Follow the stairs to a room with a bed and a workbench.
Go back up to the main hall and enter the middle door to your left. Inside you'll be addressed by a man over a speaker, a Dr. Knight, who invites you to visit him in his lab elsewhere in the bunker. Continue through this room to find an art gallery with soft music playing. Scan the wall to the left of the big green double-doors to find another weird code hidden on a painting, then go through the door to your left. Take the stairs here.
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If you scan on this basement floor you'll pick up zombies, but they're barricaded away and no threat to you. Go inside the bathroom and check the stalls for a P226 Pistol, which is silenced—a great way to pick off zombies at a distance without alerting a whole horde, if a little weak.
Go back up the stairs and through the next door. In this next corridor, follow the red light and unbar the door leading back into the main hall of the Underground. Then turn around and head into the blue-lit warehouse area. From the catwalks, you can see zombies milling about on the ground below, though they can't get to you. (For fun, you can enrage them into trying to get through the bathroom vents by shining your light at them, though they'll not be able to break through the bathroom doors.) Check the lockers on the right for Handgun Ammo, then head through the far left door. After a weird confrontation with caged zombies, you'll find Dr. Knight's secluded lab. Check the only open room here for Queen's Letters and Newspapers.
After some conversation, Dr. Knight will ask you to fetch a book from the library—a library that's exposed to zombies at the moment. Once you've picked up your next mission objective, check the rear of the room for a path to a manhole that leads back to your Safe House. Use it to save and rest up—you'll need your strength to access Buckingham Palace's upper quarters.
Once you've killed any Survivor Zombies that may have strolled into your Safe House since your last visit, dropped off any unnecessary items and saved, return to Buckingham Palace Underground and head for the elevator, unbarring the door behind the exit Dr. Knight opens for you. Go back to the art gallery and approach the green double-doors. They will open, and carry you to the upper floors of Buckingham Palace.
The blacklight codes you've been finding on paintings are for locked doors scattered throughout the game. Specifically, these codes will open doors in Buckingham Palace itself. Keep going and you'll find the door, and behind it some hidden goodies to make your life easier. More on that in the next article.