I wouldn't bother with POPLA. They will agree with UKPC that the PCs was issued correctly.
As mentioned earlier, this will go through the usual debt collector letters, which you already know, should be completely ignored as they are powerless to do anything, and the inevitable LoC followed by an N1SDT county court claim for the alleged debt.
UKPC will either issue it themselves or, more likely, use DCB Legal. They did try themselves for a period but their intellectual malnourishment in their so called "legal department" was so atrociously bad, they soon realised that it was more cost effective for them to use an experience roboclaim firm of loser legals.
A claim from UKPC/DCB Legal is simply defended using the template defence and is eventually discontinued by them. They only file in the hope that the defendant is low-hanging fruit on the gullible tree and will either capitulate at this stage and pay up or else FUBAR it and they will get a CCJ by default. If neither of these things happen, they will attempt to settle for a reduced amount, which you most finitely don't do and before they have to pay the hearing fee, they will discontinue. I would put money on it.