Just to make you aware, this is a BT residential customer to customer forum. Your post does not go to BT. The only BT staff are the forum moderators who do not necessarily read all the posts.
Did you ask the question... If I am not the account holder, who is?
What was the answer?
As has been explained only the account holder can cancel the account.
If you took over the account by paying the bills that does not make you the account holder. Any one can pay the bill.
BT accounts can not be taken over by another person except in certain circumstances such as the account holder becoming deceased in which case a formal process must be used to accomplish this.
But then equally, the account holder is responsible for unpaid bills, not the person paying the bills; that's because the agreement is between an account holder and a company.
so the question that needs to be answered if you are not the account holder then who is? presumably a relative but are they still active or not?
It does indeed! I've sent them screenshots of me to show I'm the account holder and scanned in original paperwork, so watch this space!