Ah - this explains some of it!
A bank under UK law has a duty of secrecy to its account holders. They should not even acknowledge that the customer has an account even to the account holders closest relatives.
This causes a problem when the account holder dies and is informed of the death by the NOK, unless they have official confirmation via a death certificate.
The bank can't just willy nilly take what a NOK says as the truth. What would happen if the account was stopped and the account holder was still alive.
On the other hand the bank is put on notice that the account holder has probably died and therefore should not allow certain transactions.
The bank wants to be sympathetic with the NOK as in 99% of the time everything will be genuine, and the bank does not want to be "difficult".....but it has to also look after its own legal position.
However, as far as "information" is concerned they are unlikely to "want" to provide any details to the NOK until they have proof that the account holder has died (via the death certificate).
They will then register the account on the back of the certificate and await the next stage, which will be to determine what other assets there are in the estate, whether there is a will or not and whether the estate is small enough to pay out to the NOK/beneficiaries without the need of probate/grant of letters of administration.
Please note that in the UK and in most countries the balance of a joint account goes to the surviving party, whether or not there is a will. NOK does not come into it, the surviving name on the account gets the money.
My advice is try and get a copy of the death certificate asap - most of the information will be provided then,