I'd start with "I am very pleased that you consistently applied GDPR legislation to my ridiculous tantrum about my dad's services"
I get it's sad your dad has alzheimers (which you mocked relentlessly last Christmas when he tried to take your meds) and you're waiting for power of attorney- ring them, chase them up, don't give the plusnet CSA grief, don't tag them, they're not going to take pity on you and cancel it, they can't, it's the law ffs). Your dad's contract is a legally binding document, power of attorney is a legally binding document and you'd be the first to instruct solicitors if your inlaws phoned and cancelled your broadband.
Get a grip you entitled greasy bauble.