I have an elderly relative whom from conversations with she says she is getting charged for broadband which she doesn't use or need.
Now, I haven't yet seen her phone bill with all this Covid-19 going about so the only other alternative would be to register with BT online using my email address using the account numbers supplied by her from her phone bill - would this be legal if i have her permission to do so?
It would be no different to her showing you the bill so don’t worry about it. Pointless you phoning or emailing as they won’t discuss her account with you
You would be better off calling BT and explaining the situation then get BT to call your relative and she can add you as an account manager. I did the same with Sky the sky representative called my mother while I was on the other call.
There is no reason why you can't do what you have suggested and access her account online. You will see what her 'package' is, but be prepared to be told to contact customer services to change it, as although you can upgrade online you can't downgrade.
I am a BT account holder and have given them permission to contact my niece, who lives at the property, for all billing or repair issues. This was done over the phone. I will try to find the number I used