There's no pot of gold there, because Gordon Brown flogged off most of our gold reserves when he was Chancellor.
Ooh, look how nicely my Budget balances ...
... sorry about plundering all the gold.
But actually, it is just a very big bank, like any other. Some important people have their accounts there (eg the Government), but individuals can have an account there too (by invitation of the Governor of the B of E).
If you mean Banks generally, banks actally hold about one eight of the money banked with them. The whole banking process works on the basis that everyone won't demand their money at the same time.