iPad not receiving emails from my btinternet email but it is from my iCloud email

Since your iPad is able to receive email from your iCloud account, the issue appears to be directly with your BT Internet account.

I suggest trying the following:

1. Try accessing your BT Internet email account using another device, and/or, by using a web browser instead of an email app. If you can't successfully access it this way, then the issue may be with your email account and you will need to remedy this with BT Internet support.
2. If you can access your emails via the first step, then the issue may be the email settings on your iPad. In this case, I would suggest that you first delete your BT Internet email account from the Mail app, and then, try adding it back again.