"It's not London" is a simple statement of fact. For example, it could be a reply to a train passenger for London who asked "Is this where we get out?" "It's no London" could be a dialect expression for "It's not London", or it could be a colloquial expression meaning "[This place] does not compare with [or --- is not as good as ] London [for example in the number of theatres ]."

It's no London is actually ok in context. New York is a great city but it's no London. That is, New Yorks isn't as good as London. Similarly from memory and Wiki. "Senator, you're no Jack Kennedy" was a remark made during the 1988 United States vice-presidential debate by Democratic vice-presidential candidate Senator Lloyd Bentsen to Republican vice-presidential candidate Senator Dan Quayle. Dan Quayle, the epitome of the USA!

She might be a could actor, but she's no Meryl Streep. Kenneth explains this well.

While we're on the subject if you want a laugh. America elected this bloke. A good argument against democracy! Throughout his time as vice president, Quayle was widely ridiculed in the media and by many in the general public, both in the U.S. and overseas, as an intellectual lightweight and general incompetent. Contributing greatly to the perception of Quayle's incompetence was his tendency to make public statements that were either self-contradictory and confused ("The Holocaust was an obscene period in our nation's history.… No, not our nation's, but in World War II. I mean, we all lived in this century. I didn't live in this century, but in this century's history"), impossible and confused ("I have made good judgments in the past. I have made good judgments in the future") or just confused, as when he addressed the United Negro College Fund, whose slogan is "A mind is a terrible thing to waste," and said, "You take the UNCF model that what a waste it is to lose one's mind or not to have a mind is being very wasteful. How true that is."[14][15] Shortly after Bush announced the Space Exploration Initiative, which included a manned landing on Mars, Quayle was asked his thoughts on sending humans to Mars. In his response he made a series of scientifically erroneous statements: "Mars is essentially in the same orbit [as Earth]....Mars is somewhat the same distance from the Sun, which is very important. We have seen pictures where there are canals, we believe, and water. If there is water, that means there is oxygen. If oxygen, that means we can breathe."

It's not London, meaning that place is not London. You can not say "no London".