Your email is firstname.lastname@example.org and you will have a password for that. Other than the password this can not be changed.
If you have set up your BT ID as email@example.com This is what you must have selected it to be when you set it up. If you log onto MyBT and go to your profile, you can change the username and password to what ever you want it to be. It does not need to be the same as your email address and password unless you want it to be.