You get interviewed by 3-4 people. Only one or two persons will be technical, the other are clueless... You have to show that you like to take things apart and break anything in front of you, software, networks, systems, business processes. You have to show that you are willing to travel all the time or get stuck in some village near Manchester for extended period of times.

Applied online, had a short interview with the HR gatekeeper before being given six CTF challenges to complete and do a write up on. The CTFs covered topics such as basic web app hacking, source code review and crypto cracking. I completed all of the CTFs, and did some extra, finally producing a 14 page write up documenting my methodology, as well as potential mitigations. After a few days, HR sent me a canned rejection letter, which I'm fine with, just spare me the homework assignments next time.