There is no reason that you can not use IMAP on any email app that you choose.
If you have deleted the email account from the device you should stop the iPad and any other devices from checking for email. The easiest way of doing that is to turn them off if you still have the account on the devices or add a couple of characters to the email address so that it is not checking the account. If you have deleted the account and it is no longer on the device do not try and add it at this time.
Once you have done that, change the email password and check that you can access the email account via webmail (on a browser).
If that works you should wait at least an hour to allow the servers to sync and then add the new password to the iPad, remembering to remove any characters you may have added and again check it works. If it does then move onto the next device if you have one.
When entering the settings, make sure that in the Outgoing server you do not use the "use same as incoming" for the email address and password. Enter them manually.
If the above does not resolve the problem, when you change your BT password on an iPhone/iPad/Apple device it may only change for INCOMING mail.
To change the outgoing mail password you have to go into Settings; then scroll down to Passwords & Accounts or on older devices Mail, Contacts, Calendars. From there select the BT email account and select the account where it is listed under IMAP.
You will find the details for the account and can change the password for the Incoming server there, the outgoing mail server only shows SMTP and mail.btinternet.com You have to tap on that Primary Server in order to find where the Outgoing password is hidden so you can update it.
When setting up the email account on the device you are best to set it up manually, using "other" and ensure that you are using the correct settings as per this link.
https://www.bt.com/help/email/manage-email-account/manual-settings/what-are-the-settings-for-outgoin...
