Without knowing any details, the forum does not generally advise a counter-claim. Britannia will presumably use DCB Legal, who usually discontinue in the face of a defended claim, but generally can’t do so if there is a counter-claim attached to the defence. Is it more important to you to make the claim go away, or is it more important to extract something from them?
It is unlikely any court would deem sending letters under a contract entered into by parking as causing emotional distress. If they claim you owe them money, you either pay or defend.