It's always "the UK", never just "UK". "The United Kingdom." "Been visited" is passive voice: the action happened to you. Therefore you're saying that the UK came to visit you! Proper examples: Have you ever been to the UK? Have you (ever) visited the UK?

Have you ever been in .............?

you can ask Have you been to UK? Or ...Have you visited UK before? Or ..Have you ever been to UK?

u can ask like this : Have u been in U.K ? well use it for informal question such as asking friend, etc