Many years ago (around a quarter of a century!) I could pay money into my own account, or into anyone else's bank account, simply by going into any High Street bank and filling in a 'bank giro credit' form. Alas, despite so-called 'progress', that facility was withdrawn many years ago, so your father will need to visit a branch of your bank. (Theoretically it's still possible to use any other bank branch but there's a hefty charge, typically of around 30 quid, to do so).
Some inter-bank transfers can be achieved simply by visiting any Post Office branch but, while it's possible for your father to both withdraw and pay in money using his own account at any Post Office (because he's with Lloyds), it's not possiblwe to pay into a Nationwide account there (even though you can withdraw money there yourself).