There should be three steps for adding an email service to a Fire tablet.
- Open the Mail app on the fire
- tap the "add account" option
- enter the email address and the associated email address password.
You should then get the small "spiral of death" symbol in the center of the screen as it goes away and checks everything out, and then it comes back with Account Added.
If you have more than one email address account then in the schedule on the left of the screen you will see each account inbox and can select them.
I regret that whenever I see "BT" on an Amazon forum I expect to hear problems - something is always not quite right it seems and it's never clear if the problem is Amazon or BT. It may well be worth going to the BT Help forums and searching for "adding my mail to a FIRE" or something and seeing if there are others with the same problems who have worked out a simple and straightforward solution.
(That's about the limit of my advice :( - if there are helpful or even unhelpful messages coming back like "server not available" or something then maybe advise and someone with more technical skills may be able to guide you!)