I went through UK security clearance and can tell you that the process is to determine whether your personal/sexual habits are likely to make you vulnerable to blackmail. Not so that the government or agency can blackmail you.

That is: If you happen to be into BDSM and this is something you keep extremely private, then what would you rather give up? Your personal privacy or a piece of sensitive data?

So the interviews at different levels determine whether you can be trusted with the information based on the risk you pose to factors such as blackmail, or financial rewards, etc.

Turing died because his lifestyle was illegal and he was being punished for it, not because it was a secret.

Turing died because he had a head full of military secrets and consorted with gay prostitutes, and was therefore considered a security (blackmail) risk.

Obviously it was more complicated than that, but I consider this to be the primary reason.

Interesting theory you've got there. Please explain why, if he was considered such a security risk, he was charged and tried publicly in a civilian court, and then allowed to roam freely for two years after his conviction.

I think I'm coming across as more of a conspiracy theorist here than I intended.

When he was convicted for homosexuality, it would have come to the attention of various senior people in government. They would have wanted to make an example of him, being as he was a relatively well known figure at the time, much like what happened with Oscar Wilde.

They would have also had concerns about his homosexuality being used against him by britain's enemies, and also because of the recent uncovering of a ring of soviet spies who were all drawn from an intellectual set, all academics, and two of whom were gay.

What I am saying is, I find it likely that these government/military figures would have found it highly convenient for Turing to disappear, or at least have his life made extremely difficult for him. They couldn't have him executed for his sexuality, but they could apply chemical castration. They did offer him the choice between castration and prison, and I accept that this weakens my case. You might ask, if they were that concerned, why didn't they just have him conveniently die in a car crash? My answer would be that they weren't that concerned, it was an opportunistic thing.

Also to be honest I am not entirely convinced that his death by poisoned apple was necessarily suicide, but there is no evidence for that, and there never will be.

It is call a "Lifestyle" Polygraph and it is meant to weed out anything that can be used against you in exchange for divulging secrets. The major component to most of the clearance questions revolve around bribery. People don't divulge secrets typically because they hate the government or the country, but for much more simple reasons like they need to pay rent and they figure they can sell some secrets.