1. It's not called the Metro in London. It's called the London Underground, or just "the underground" or "the tube".
2. The London Underground runs all across London so you can't buy a ticket on it "to London". That makes no sense. You're clearly already in London.
3. You can use "to" if you're specifying an Underground station or an area of London. For example, "I bought a tube ticket to Leicester Square".
Some parts of the London Underground do run overground but it's still called the London Underground.
The London Underground is a completely separate company from the various overground train services that run through London and across the rest of the country.