The difficulty you have is that, legally, it's not your account and you have no basis for accessing it unless the account holder provides it.
The payments are a different matter and you could freely stop payments from your bank account with no legal consequences to yourself. It is the account holder who is responsible for payments, not someone who is basically a third party
The correct solution in this scenario would be a legal change of ownership so you are responsible - and have account access - for a phone that, presumably, you're using. That will require the consent of the current account holder - see "transfer to a friend, colleague or family member" under https://ee.co.uk/help/profile/manage/change-my-account-in-the-case-of-a-life-event?
