If your sub account is on the BTYahoo email system and your own BT account is on the BTMail system I think you will find that you will not be able to have the sub account moved onto your BT email account and made the primary email because they are on two different email systems.
I believe you should however be able to have the sub account "associated" with your BT account which would take control of it away from your fathers's account. You can not do this your self and needs BT to do it for you.
I have notified the moderators of the forum about your problem. Once they have read this they may be able to help. They are a BT UK based team and if they can help they will reply via this thread asking you to contact them via a link. They are very busy at present so once you have replied to them by the link, it can take over 48 hours for them to re-contact you.
Welcome to the forum and thanks for your post!
You can send us over your details so that we can check if we're able to arrange this for you from here. Click on my username here >> RobbieMac << and you'll see our contact link on the left hand side underneath my profile picture.