I just wanted to say that I have spent two days trying to get either Outlook for Android or Samsung Mail to recognise the BT email IMAP account. I kept getting a message saying that the account and password weren't recognised even though I knew they were working on webmail. I eventually found a post on the Microsoft support site for Outlook and it said make sure that you don't have ANY special characters in your password (BT's help pages just say you can't use a £, - and Euro symbol). It turns out that this was the very simple answer - I changed my password to just letters and numbers and Outlook immediately recognised it and it is working fine.
BT are aware that the £ and ¬ (logical NOT) character are accepted in their set and change password screens.
Passwords containing these characters will work with webmail and BTID logins, however because these characters are not part of the US ASCII character set, they will typically not work with IMAP email clients (this is a limitation of the IMAP specifications).
BT will be addressing this in an upcoming change to their password screens, and in the mean time BT recommend avoiding these characters in passwords for email accounts.