The million dollar question that's asked here often but to be fair to NowTv, it's not just them who make it difficult. There are a few theories but the long and short of it will be that they don't want you to leave! No subscription based service does. As long as they have the information, they can send you offers and attempt to lure you into purchasing their product.
I'll caution you, it's a very long read. The first two things that stand out are what I suspected - that financial details are kept for 2 years and that they expressly state that although you have a right to erasure they have the legal right to keep some information (the legalese I mentioned)
"Please note that some of these rights are not absolute.....if we had a legal obligation or right not to comply with the request."
The stand out section for me is how long they keep our information - 7 years! That's a long time for them to try and lure you back.
If I was in your position I'd bypass customer services. No disrespect to the guys and girls sat in an office in India but I doubt they have any idea what GDPR stands for let alone know what 'right to erasure' is. There's an email address for a department that deals with privacy issues so you're probably best emailing them and reminding them of their duty to action your request within 30 days!
"I highly doubt Sky would make the mistake of keeping data illegally"
As if you're suggesting Sky don't make mistakes like this. Sky have a proven track record of sailing close to the line and I personally know of a number of people who have had issues with Sky that are unethical and borderline illegal.