I was contacted by a recruiter that proposed me the job offer, so I applied. After 3 days we arranged a telephone interview that last 30 / 40 mins. First part of the interview was related to past projects I worked on and tech solutions I created to solve problems in those projects. Then we started a tech interview about manual memory management (retain, release, everything before ARC...), grand central dispatch and queues management. Also, they asked some questions about memory allocation (stack, heap and so on). After a week we arranged an "on-site" interview in London, Finsbury Square. There were two guys, the interview last for 2 hours and a half. Lots of "exercises", such as writing algorithms on paper (e.g.: Fibonacci, quick sort, etc...). They also asked a lot of questions on "standard C" (this is the step that I probably failed).

Similar interview process to the rest of the tech companies like facebook, google, etc. A few technical interviews, system design, behavioral. On the technical interview they asked how I'd change a piece of code they provided. Very polite and welcoming HR people.