If you are a business user you will need to post on the BT Business forum where you should be able to get some help.
See link about BT Premium email
I am not pinging you. This is a BT residential forum and the BT moderators are unable to help any BT business user accounts hence the reason I suggested trying the BT Business forum where the moderators can assist BT business customers.
Right, just call the premium mail team, don't even mention you are a business customer as that is totally irrelevant if it just that you want to keep an email@example.com email address without a BT broadband account. It will cost you £7.50 to keep the address.
Try 0808 100 6778
Edit: If you want to transfer your firstname.lastname@example.org email address to your business account, I doubt if that is possible.