I was in a similar (yet a reverse) situation. As a Turkish citizen, I applied for the Istanbul office, and since I was based in London, I had my first round in the London office (via a videoconference with the Turkish team).
You will most likely be invited to do the first round at the Istanbul office in Kanyon, and will speak with the London team via a videoconference. Surely, if you prefer to fly to London to take the interviews, you may do this at your own expense. BCG typically flies candidates in for the final stage of interviews.