At the behest of Sondra of the Ravens of Dee you're now at the Tower of London. If you haven't already, check the door to your right for the manhole back to the Safe House. At the top of a ladder here is a door you can't yet open, so ignore it and go back to Traitor's Gate.
The normal path into the Tower is blocked by an electronic door you can't open without a Key Card. Check the left side of the stairs for a small hole you can crawl through. Attack and kill the zombie slumped against the wall here before it can get up, then slide through and into the underground. There's Juice on the ground; ignore it or grab it, then head into the dark.
Down this hall you'll hear some groaning, and see the shadow of a zombie down the hall. Creep into the room without alerting this zombie, as it's not alone. You'll be attacked by another zombie from one of the cells. Kill it, then climb through the gap in the right wall. On the other side you'll find a CCTV junction box you can scan. Creep up behind the zombies from here and you can more easily lure them one at a time. Once they're dead, unlock the door here and go up the steps beyond. A body here has Handgun Ammo.
Continue along into the next room. You'll find a strange black-lit wall inside, and Sondra will explain the symbols' significance. Scan it to open the door and enter. Within you'll find another waterway, and if you scan the area you'll see arrows leading you onward. Go up the steps to find a hole above the gate that you can fall through.
Proceed forward and take a left. Not because this is the way to go, but because an exploding zombie will show up from the right. Get back and take it out before it blows. Going further left will alert another zombie; wait for it to run over to you and kill it likewise. Go right this time and, with a bit of exploring, you'll find a third zombie. Once it's dead you have free reign of the place; follow the arrows through the waterway to the end. Make sure you check the very back of the area, near a grating, to find a Capacity Upgrade on a small ledge.
Follow the arrows until you find a symbol on a wall. Hack it, then get ready - you're about to face three zombies. Lure one into the water so you don't go too far into the room, kill it, then go into the room and get the second. The third will crawl in; leave it until last. Once they're dead, go up the stairs.
Check the bodies here for Handgun Ammo, then go through the crawl space at the end of the hall. Back out as a zombie comes for you, kill it, and go through again. You'll come out in the 'nerve center' of the Ravens of Dee. Check the cabinet by the door for The Black Prophecy, a seat at the back wall for another Prophecy, and a suit of armour in the corner for a Spread Upgrade. Most notable of the lot is the AK-47 Assault Rifle propped near the flat grey door.
That done, unbar the door into the room and scan the sigil on the back right wall. You'll discover a cipher for those weird codes you've been finding all over the place. Scan the room to find arrows pointing at a cabinet, and shove the cabinet aside to find a code. Use the scanner to watch the cipher in action, then use that code to get through the Exit door in the back.
Once up here you'll start to hear bells tolling, and you'll see a bunch of zombies. Sondra will help you out by killing a bunch of them with her sniper rifle, but she can't get them all - and you should be ignoring this anyway. Instead, run for the stairs to your right and back, away from the front door. Go up the stairs, wait for a zombie to burst through the door, kill it, and run through.
The next part is pure action. You have to race through the ramparts, killing zombies along the way. Murder everything you come across and make your way to the top. Most of the zombies are normal, though there's one spitter on the way. Make it up to the helicopter and hold out for a few seconds -
- only to watch the thing crash in the most ironic way. Great. Sondra, distraught, will promise to find another helicopter, and she tells you to stay alive. After she leaves, kill two zombies that come up towards you through a nearby door, then go through said door to find the path back to the manhole. Return to the Safe House for a new mission from Prepper.
(Don't worry if you die here. The story will go forward without you having to make a trip all the way back to the Tower of London to do the action sequence again - though you will still have to fetch your gear from your old survivor.)
Return to the Tower
If you felt rushed during the escape sequence, you probably didn't take much time to look around the rooms of the ramparts. In that case you may want to go back to loot the place, as there are a few items to grab along the way. In the first room you'll find some Assault Rifle Ammo, in the second there's more Assault Rifle Ammo, a Double-Barrelled Shotgun is in the attic, and an Energy Drink is in a container on the roof. The area is much safer this time around, though there will still be zombies in the ramparts to deal with.