ZombiU walkthrough, Part Six: Buckingham Palace Gates
(Zombies from online players are fairly common in this area. Don't let your guard down.)
After leaving the small surveillance room, check the area behind you. You'll find a Rubbish Bin behind the ATV with some Handgun Ammo, as well as a bunch of Rats. Don't try eating the rats for health unless you're really desperate - some will undoubtedly hurt you instead. Another body on the opposite side of this clearing has more Handgun Ammo. Once you've collected your fill, proceed towards the gates in the distance.
When you get close enough you'll be attacked by two zombies, one walking and one crawling. Back away to deal with the walker, then close in and squish the crawler. The crawler has Handgun Ammo. Once they're dead, check to your left to find a small area with a container and a body. The body will come to life; kill it and check the corpse for Juice. The container holds Carbine Ammo and Handgun Ammo.
Move close to the yellow ambulance in the distance and a new kind of zombie will come at you: the SWAT zombie. In order to kill these suckers you need to first knock off their helmet. Dealing with a SWAT zombie is no different than a normal zombie, though it will take several more hits of the Cricket Bat to take off the helmet, and guns generally aren't quite as effective. Try to isolate SWAT zombies from others. After it's gone, look to the containers behind the ambulance for another zombie. Kill it and steal the Flare from its body.
If you check to the left of the ambulance you'll find a locked door. This path is your way into Buckingham Palace, and you need a Lockpick to get in. Continue past the flaming helicopter wreck to find the gates proper; in this area you're only likely to see one zombie, creeping around some containers to your right, though there are two more waiting to rise up, including one SWAT zombie. Try to lure the first zombie across the clearing to you to roust the other two out of hiding, then kill them one at a time, leaving the SWAT zombie for last. The SWAT zombie has a Medium Medipack.
You can't just stroll through the gates, so Prepper advises you to look for a little hideout nearby. First, check a box near the gates for a Flare, then look at the fixed turret emplacement on the right side for Fixed Turret Ammo. You may want to leave this here - you'll need it soon. Go through the nearby container to find the hideout Prepper spoke of, out front of which is a Metal Container bearing two Fixed Turret Ammos, Handgun Ammo, two Grenades and the Lockpicking Tools. Before you snag any of this stuff, though, check the hideout behind you for a place to sleep, as well as a workbench for upgrading your weapons.
Grab at least one of the Fixed Turret Ammo boxes and a Grenade, then take the Lockpicking Tools. As soon as you do a zombie will rush through the nearby container. Kill it, then head out to the fixed turret and get ready for a horde of zombies, as a ton of them are about to surge towards your position. Using the turret via your game pad, you'll have to kill them off-one-by-one. The majority of this section isn't so bad; just aim high and fire with care. You should be able to kill most of them with a quick burst to the head.
Unfortunately, this part is ended by the arrival of two SWAT zombies that don't care a fig about your turret. You can deal with them normally, but your best option is to drop a Flare, wait for them to rush over to it, then toss one of the Grenades you picked up at them. This will take them out in an instant and end the battle. If you do get killed during this fight, you'll keep the Lockpicking Tools - but you'll find a bunch of roaming zombies in this area when you come back, including your old survivor.
Win or lose, you can now open the way to the sewers under Buckingham Palace. Go back to the locked door, jimmy it open, and go down the ladder in the nearby manhole. Check behind the wall down here, as there's a zombie lurking less than five feet away. Kill it, then go through the door and back into the sewer system of London.
The Fixed Turret
Using the fixed turret is a little misleading at first, as you're supplied with a ton of rounds. Don't go crazy firing at the zombies! The wave here is fairly long, and you will slowly run out of ammo by the end. This makes bringing boxes of ammo with you all the more important. Always aim high, which isn't too hard since the aiming reticule is nice and steady. Sweep the entire area for zombies, as well - they come from the left and right in equal measures.