The Leopold was a big, 11 inch caliber cannon. But it's nothing compared to Mallet's Mortar, the largest caliber gun ever built (36 inch).
I believe that there are two Mallet's Mortars left. So the Royal Army has considerable backup firepower if needed.
![](http://www.royalarsenalwoolwich.org.uk/assets/images/Mallet_s_Mortar.jpg)