I found the list of locations at: Are the C4 explosives limited? - ZombiU Answers for Wii U - GameFAQs
My question now is which 1 location where I can use C4 should I skip.There's a total of 7 C-4 in the game:
5 at the Military Armoury
1 at the keycode room in the Nursery basement
1 in a locked room in the sewers of St. George's Church
If you get all of them, I think it still leaves you 1 short if you tried to blow up all of the C-4 spots.
1. Green Park at the swamp
2. Tower of London Gates
3. Dee's letter in Brick Lane
4. One of the Warehouse rooms in Brick Lane Flats leading to a Red Zombie and upgrade
5. Basement of St. George's Church by the bathrooms
6. By the entrance of Spitalfields park to get the XXL backpack
7. After keypad door in Buckingham Palace Kitchens
8. Basement of Royal Bunker during escape mission (after the exploding zombie by the bathrooms)
The location is at St. George's Church in the starting area right after you go out of the door leading to the church catacombs. At the first pushable shelf, there is a hole you can crawl in and you'll find a dead body and a suitcase. The suitcase contains the C4.