There is, of course, a more disturbing possibility. A number of VM’s customer service agents are corrupt and are fraudulently adding various ‘extras’ onto customers' accounts so as to boost their bonuses for ‘upselling’ services. See the various threads where customers claim that ‘Homeworks’ has been added to their accounts without their knowledge.
Of course, I wouldn’t want to suggest that VM knowingly encourage their customer service people to do this, it’s more the fact that they have outsourced everything to the lowest possible bidder and turned something of a blind eye to what they do and washed their hands of their actions.
So, here’s what you need to do. VM claim that you authorised them to add Netflix to your account, the onus is on them to prove it. You first request a DSAR, even requesting, no, demanding this costs them money and they are legally obliged to do so. Simultaneously, you raise a formal complaint - be aware that this complaint will go nowhere and will be answered with a meaningless response, apparently written by a three-year old, and will end with ‘we hope this works for you’.
Fine, reject the ‘resolution’ wait eight weeks and raise a complaint with the Ombudsman service, in which you request that VM cancel the (fraudulent) Netflix add on, and refund you all the extra monies they have charged you, plus a compensationary sum of three times what they have (over) charged you in recompense for your time spend progressing this, their failure to provide proper service etc.