There's nothing required by law, as you put it, that a guarantor must provide. This is a contract between two or more parties and you or your father must provide enough information to convince the landlord that you can afford the rent and other liabilities.
As a landlord I would get a credit check done on the tenant and also a reference from the employer and, if applicable, from a previous landlord. If a guarantor is required then then same checks would apply to him/her as well. Some landlords ask for bank statements as well.
Bank account numbers and sort codes are fairly irrelevant, both to the landlord and to anyone else. Blank them out on the statements if you wish but there's little point really. As has been said, they're printed on cheques etc anyway and are no use to anyone unless someone is paying money into your account. Perhaps he's more concerned about the personal information actually on the statements (incomings and outgoings) rather than his bank account details.