I agree with Michael.(posted 7005 days ago)I have sent many SARNs in the past and frequently used postal orders - a form of money, in effect - and they were always accepted by banks including, for example, Abbey National.
Please would you be kind enough to send me a copy of the letter you received from B&W? I can be contacted at Members', Portsmouth City Council, Guildhall Square, Portsmouth PO1 2AL.
If B&W are denying the legal and financial status of postal orders, them I think that we, the Post Office and the Treasury need to know about it.