Banks generally prefer you to store your gold 'unallocated', so that it becomes formally their property. Because gold qualifies as a liquid reserve asset, which underpins the bank's ability to lend, what you think of as your gold will actually permit the bank to expand its balance sheet.
It does this by taking more short term cash deposits, and lending them long. It's the ability of your gold to convert quickly into cash (to repay short term depositors in a hurry) which allows the bank to do this.
Banks can expand their lending by about eight dollars for every dollar of their reserve, so they make a lot more money out of doing this than they do out of renting you vault space. That's why they tend to offer you 'free' unallocated storage, but very costly allocated storage, at something like 10 times the wholesale rate for vault space.
Of course with unallocated storage it would be your gold (technically their gold) which would be sold for the benefit of the bank's short term creditors during a bank run. Not many gold owners would elect for that option if they understood it better.
Commercial vaults - including those used by BullionVault - have no banking licence and no motivation to over-price allocated storage.