DVLA are terrible, they sent me a letter yonks ago about my car not being SORN and they would fine me, so I sent them a lovely letter explaining how incompetent as they are and they
ones who lost the letter and I sent them a copy of the sorn application I filled out before hand, to which they dropped their nonsense and SORN'd my car which should of been done ages ago.
But as Elvi has said, you can always do an affidavit to acknowledge thats you served the papers via a county court or a commissioner of oaths.
Also demanding money is one thing, another is taking it to a court and getting an order for the bailiffs to instruct on or enforce.