I am confused about the question below. Why does the "mı" come before the "yaşıyorsunuz"? Why isn´t it: "Siz Londra´da yaşiyor musunuz"?

Siz Londra´da mı yaşıyorsunuz? 

Your suggestion is correct also but the questioner here wants to give emphasis to the place. Just like he wants to say he knows you live somewhere  -  but where, that is the issue. The question particle mI can be used for questioning a constituent as well as a whole sentence. Look here also:

                     http://www.turkishclass.com/forumTitle_51637, post 7 by si++

This makes sense. I just had not seen this in any of my lessons yet. ... Thank you, Abla.

Addition to Abla´s post;

If you say "Siz Londra´da yaşıyor musunuz?" that may sound funny or weird because it is like asking:

"The action you do in London is "to live", right? You are not dying or doing anything in London, correct?"

The reason is that "mı" suffix comes after the word you would emphasize.

Gidiyor musun? Are you leaving? (or maybe you are not leaving you will just step outside to get some fresh air and you will come back some minutes later? etc.. asking the action)

Dışarı mı gidiyorsun? Are you going outside? (or maybe you are going another place not outside?.. asking the place where the person goes. but the fact is the person goes. you are not asking if he goes or not)

Dışarı, hava almaya mı gidiyorsun? Are you going out to get fresh air? (asking the reason of going out is "to get fresh air" or not. the question is not about "going out" because it is not obscure, that is clear he is going out. but the unknown part is the reason)

So the emphasize (the real question) is on the word before "mı" suffix in the question sentences.

Thank you! This makes it even clearer. But the lessons so far on the site have not made this point at all (I am starting Beginner, Section 3). Which makes it hard to do the exercises. I think I have come across two questions now that use this construction, but I thought it was wrong because I had not seen it mentioned in the lessons. ... Confusing for a beginner. ... Nevertheless, thank you to you both. I will now recognize what is going on here.

The lessons only give you a start. Real learning begins after the lessons and it has no end.

Abla, this is the same question i´ve asked you in my exercise that you evaluated to me remember

Yes dear. And I told you it is always better to ask in the forum. You get better answers from natives and more people will benefit. Remember?

yes i do, since that i´m reading every entry in the forums. Abla you rock