One route would be for you to tell the Bullshit Purveyors and DVLA that you like being ignored and fobbed off in a condescending way. That's a service that they are both especially good at.
However, if it was me, I would send an electronic copy of the initial 'no I won't give you any money' along with a scan of the POD and make it clear that it is no use them lying about not receiving your correspondence and you want your POPLA code by return (scrub that, you f
cking well want it!). This is why I discourage the use of snail mail when brushing off parking weasels. It's a complete waste of time, effort and postage charges. Electronicalthingammybobs are indisputable. I know this because I already have my UKPC Ltd POPLA code.