Yahoo terminated all local domain extensions like .co.uk to only keep the .com extension. It is no longer possible to create an account with a local domain extension. However, accounts created before the termination will continue to work without any problem.
If you want to sign in to your Yahoo account ending with .co.uk, all you have to do is to enter your full email address manually on the sign in page. Note that if your account hasn't been used for more than a year, then it has been deleted due to inactivity.