When you die before looting your belongings from your former zombie self your weapons and items will respawn in previously visited areas. Use the computer in the safehouse to scan the CCTV junction boxes to find out where your items have respawned.
The CCTV boxes can be found at the following locations.
Tower of London: In the area where you drop down into before you unlock the secret passage go to the end of the passage behind you before picking the lock on the gate. You will find the box there on the left-hand wall.
Buckingham Palace: Use C4 to blow up the wall in the underground section. The box is next to where you find the pistol with the silencer.
Green Park: The box is to the left of top island. Going right will lead you to the tunnel that takes you to the piers at the Tower of London.
Bricklane Flats: The box is behind a wall you must destroy with C4.