I have sent the following email to the Post Office:

"My local Post Offices refuse to give me a receipt for receiving my
money so I can put it with my accounting records so I can correctly
prepare my tax return and have evidence of the payment if HMRC wish to
check.

They say that I can use the receipt of postage instead. I need the
receipt of postage kept with the letter to resolve any dispute with
the recipient.

Please explain why the Post Office refuses to provide receipts for
money received and wishes to inconvenience their customers."