No you don't. If you deliver something by mail you have no guarantee they got it, unless you use a service that requires a signature on delivery. Even then all you might have is a guarantee that *someone* signed for it, not that the person it was addressed to did. Our posties often just sign for things if it's something they can fit through the letterbox and no-one answers.
But then in the UK you don't have to prove they got it, just that you sent it. Proof of postage doesn't cost you anything; you just have to take it to a post office and ask for it rather than using a postbox.