In truth CG I don't think you could have access to a joint account simply by being the apointee of one of the account holders.
The bank could(should?) be advising you to set up a new, separate account, it could be in your name, it could be your own account, into which your Mum's State benefits are paid.
In view of the fact that I have just received £516 as my half share from a joint account with my estranged husband from an account I took my name off 13 years ago, as compensation for mis-selling Personal Income Proctection on a loan, he took out and re-paid on the dot.....
I would think a savvie bank is going to be very careful about how accounts are operated and I fully understand their reasoning, in fact they have been lax in their use of the rules for themselves and lax in their proper obligation to apply the rules as regards customers.
I don't think there should be descretionary decisions and rules waived.
Accounts should be frozen if legal obligations are not fully met, the bank could come to an informal lending agreement with customers to allow low-fee lending to be granted, perhaps even statutory limits where it is obligatory rather than credit-check or history linked, while problems are ironed out, fees for lending disregarded once disputes are clear and a low adminstration fee paid by the customer for the service, or the customer gets off their backside and sorts their finances out for themselves.
It might mean more upheaval but better things are set on a legal footing, as they should be, than open to interpretation and mis-use by any party concerned.
Single accounts with mandates to pay into a joint account are much better than a joint account alone, where everything, debits and credits are jointly liable but ability to operate the account could be ignored to the detriment of a vulnerable co-signee or suddenly ended by the legal requirements of a joint account holder.
I think all kids should be taught the in's and out's of banking and the basics of law with regards to money and there would not be the ability of the banks to run amok over naive customers nor would they be the naive claims about the freezing of accounts when a person lacks culpability or ability that causes so much pain when the inevitable comes to call. IMO.
Form BF57 is I think, a form about apointeeship to State benefits and pensions and applies to a joint bank account insomuch that it means that money is accepted to the account, to be shared by both parties as a credit, as are debits but as an appointee you are basically accepting receipt of the money, nothing more.