Other countries already hosting a share of the World's Wealth could make room for more. New York vaults for example are capable of housing more than they have. So dividing up the 1/5 that's housed in England should not be a problem. Foreign countries pay a nomimal movement fee to the storage facility for transfers from one country to another. Storage of such wealth is ongoing and lucrative.
Security vaults are not the only way to store wealth. It is also important to remember that there is only a finate amount of things on this planet and their is certainly enough space to store them, even if not all of the things can be kept as securely as they once were.
Most people will do anything of money, so will most countries. If it concerns their wealth and gold, they will always find a way to keep it safe. If the vaults become full, they will build a new one. If their security is compromised, they will update it. There's not much thought to it.
There are plenty of vaults across the world that can handle a large amount of gold. I mean, 80% of it is already not held inside the vault, so I don't see what the question here even is. Maybe we can put some back at Fort Knox since we haven't used it for that in a while.