They are both correct but it depends on what context. I have been in London.>> A good answer if you've been asked, where have you been the last two weeks on your trip? I have been to London.>> A statement of fact as to a place you have visited. Or an answer to a question like: Where in England have you ever visited?

I have been in London

We don't use "variant" in the same way Russian speakers use it. "Which version/phrase/sentence/option is correct?" If you are talking about travel (ie. transit), then "to" is the correct preposition. If you are talking about a fixed location, then "in" is correct. In most cases, you will probably use "I've been to London." (=I've gone there, and have come back.)