Hey, I was asked this very question at another firm and I successfully answered it through discussing three main reasons:
Firstly, i would recommend focusing on London office exclusive teams, do they have a particular offering only in the London office which you find attractive or a particular way of working/structuring an offering? This would also be good to show off your research into the firm and you can talk about what makes the firm unique to competitors.
I would also talk about that offices specific culture, have you spoken to any employees or read any testimonials about the London office’s culture which you found attractive and could draw upon? Is the London office larger than their other offices and thus good because of proximity to higher mandate deals or is it smaller and also good as you will get to know everyone and be more than just a face etc.
And then finally simply talk about the normal reasons, do you live in london already and therefore call it home, or would you want to live there? Is it more convenient for you to commute to london than any other office? Etc
Hope this helps! Best of luck with your AC!