At some point in the game you will make your way into Buckingham Palace, it's story required. When you are there you will find a lab scientist who asks you to find a book in the palace. Once you find that book a little cut-scene will play where a small cupboard type door will open revealing a small hidden path. If you enter the path and make an immediate RIGHT turn you will shorty find a dead-end with one zombie and the backpack. Dispatch the zombie and grab the backpack.
If you wait 'till later in the game when you get the XXL bag you may be unable to grab this one after that point so get this one first just to be safe.