I have been having problem logging into my email. Even though I know the password and that is what I have been using from a long time. But now when I try to login to my email it says incorrect username or password. I am not able to reset my password online as I have lost access to my alternative email as well. I think my email might have got compromised. Before I call I just want to make sure that I am calling the right number.
Thank You.

all you have to do is call it, and see if it is right ?
but I show from web search, these three numbers
but, there should be a "forgot" passcode to allow you to reset

800) 318-0612
800-730-2669
800-405-7988

Supposedly that is the real Yahoo support number for the UK.
When your email is compromised, one good thing to do is to contact friends and ask them what happens when they email you.
I don't think I know anyone who hasn't had their yahoo email hijacked, including me. I switched to gmail, which has far better protection.