I have set up forwarding in a Gmail account so that my messages are forwarded to my main BT Yahoo Account.
Everything is working on Gmail's side, but when I tested it by sending a few emails from the Gmail account to itself, these did not appear in the BT account.
Can you please tell me why the BT email is not accepting forwarded mail?
Have you tried sending an email from another email account to your gmail account to see if it gets forwarded?
You could even try using your BTYahoo email account if you don't have another account to use.