Well, I do not know the law in UK but here the official money is a lawful payment and valid currency to pay legally binding purchases. As a business owner you have to make it perfectly clear how you want to have a contract ( i.e. selling goods or services ) with a client without money. In advance when you do not want cash. You cannot agree upon a sale and afterwards refuse cash as payment. But you can negotiate some other payment terms. When you do not negotiate something else in advance but cash you break the contract which could lead to a fine.