You still don't get it do you ? By the time it's got to the stage where you are having ping pong with some silly debt company it's already too late. You ignore them completely. Zero communication. That means no letters, no door step conversations, no angry shouting matches on the phone. Why are you wasting your time reading entrapment legislation ?
That all depends on the legitimacy of the debt.
Ignoring a fixed penalty notice worth £60 could cost you £600.
A parking charge notice: worth a punt.