I don`t think that is possible unless you are legally appointed to deal with the affairs of a person who has lost mental capacity.
I will ask a moderator to comment.
Doesn't work. I tried to set up a BTID with my email address, it says that I already have one. I log into the account and it takes me to the webmail account. When i click on the "My BT" link it takes me straight back to the login page. I log in, it takes me to webmail. I click on the "My BT" link, it takes me back to the login page..................you get the idea. It won't give me access to anything other than email, so I can't enter my father-in-law's account number and I can't find any way to manage his account. He has severe dementia and his contract is running out. Please tell me how I can get myself added as a decision maker on his account, this is soul destroying.
It might be easier in your case to set up a new BTID using a new or alternative email address. The way you have described your BTID ID acting is not normal but has been also mentioned by other users in the past.