It depends on which client you are using. Like Pidgin requires you to just use your Yahoo name, while Digsby requires you to use your Yahoo email address. Also the Yahoo IM is part of the yahoo.com website. So even though you are on BTYahoo in the UK, that doesnt mean you are eligible to use the YahooIM chat program. You will have to go to yahoo.com and sign up there to get onto their service.