It helps that British geeks love James Bond and Alan Turing, and at GCHQ they get to play at being both - although you'd have to hope some of them have wondered if this time round they might not be the unequivocal good guys.
Also, if you get a reputation for hiring the brightest and best, you also get people who've spent so long being told they're the brightest and best that they may have little concern for whatever lesser beings get spied on, shamed, discredited, disenfranchised during their fun intellectual challenges.
GCHQ have a brainwashing program that is designed to keep their staff in line with the status quo. If you so much as step into an interview, you have joined their cult.
I'd imagine many working on this won't tell you their real job. For example when you apply for MI5 stuff in the UK you cannot tell people your actual job.
