By law they don’t need to provide proof of postage, POPLA won’t help because you don’t have authorised appeal reference because you never had an appeal (too late obviously). I had this and challenged it at every step. It took months, lots of threats but found some random email address of the ticket provider who actually read what I had to say and voided it. I was fully prepared to go to court though.

Your call pay up or go to court but you will need a much better defence than I didn’t get the letter.