If you heed the advice given here, this is what is going to happen...
Wait until you receive a Letter of Claim (LoC) and then show us so we can advise on a suitable response. Eventually a claim will be issued and we will provide a defence.
Eventually, the claim will be either struck out or discontinued.
Until then, ignore all useless debt recovery letters. Debt collectors are powerless to actually do anything except to make the low-hanging fruit on the gullible tree pay up out of ignorance and fear. You can safely ignore all debt collator letters, they cannot do anything.
You cn go through the POPLA appeal process but in these cases, they are unlikely to be successful. It's you time and effort and maybe someone else on the forum will assist with a POPLA appeal. Personally, I wouldn't bother and I'd wait for the claim and defend it.