Did Yahoo change all of their “.co.uk” email addresses over to … – Reddit

If your account exists, and is still active, then e.g. william@yahoo.co.uk will work as expected. But the login system may be expecting you to type in a yahoo.com address or similar. As I explained previously, I have seen a couple of odd behaviors from Yahoo when it comes to login. Clear cache, clear cookies, be on UK IP. That should do it.

https://www.reddit.com/r/yahoo/comments/15rnuzl/did_yahoo_change_all_of_their_couk_email/