If you've already set up mail forwarding on your BT Yahoo account, you should be OK.
But Yahoo have now disabled the option to set up any new mail forwarding "temporarily". https://help.yahoo.com/kb/sln22028.html
Only the most cynical person might suggest that Yahoo are trying to make life difficult for anyone trying to migrate their email away from Yahoo after a spate of bad news stories about them.
Oh, it looks like someone has suggested it http://www.theregister.co.uk/2016/10/11/yahoo_email_lock_in/
Although I enjoy a conspiracy theory this does not really stand-up.
If you plan to move to another email provider and just want to keep your yahoo email going while your new address gets established then check what services the new provider offers. Most support a way to collect your messages from another server using POP/SMTP and it should be just as good as forwarding. As I understand it, for either forwarding or collecting emails to work you would need to keep your account active ... if anyone knows different please let me know.
Since the great data breach in Sept most people have not changed their passwords and would have no idea such a thing as automatic forwarding exists .... this would mean the downside of leaving the background forwarding option available would be to leave those users exposed to being hacked and the crims setting-up feeds of their messages un-noticed and then allowing them to steal the identities of, possibly, millions of people.
Having said all the above I think Yahoo are criminally negligent regarding atrocious data security and as the US lawyers sharpen their billion-dollar pens and prepare some bankrupting lawsuits we may all need to think about where we move our email to in future ... just a guess.