John Drake is Number Six
This theory is as old as The Prisoner, the show that replaced Danger Man. They are very similar characters (Drake is a spy, while Six is a former spy who was drugged and imprisoned after attempting to resign without saying why), though there are a few differences. Drake, for example, doesn't drink, but Six does. Drake is generally more mild-mannered than the very intensely angry Six. Of course, these could be easily explained by Drake being under intense stress due to being trapped in The Village, and the stress of his job may have been getting to him before that (hence his deciding to quit). It should be pointed out that Patrick McGoohan stated that Drake isn't Number Six but he was a bit of a contrary fellow, and anyway, what would he know?- Taking this further, Drake is also the British agent in Ice Station Zebra. The fact that that character (who is called David Jones, which is likely an alias; his real name is never revealed) is a heavy drinker could explain why Drake decided to quit — he feared that drinking, a habit he'd recently got into, was affecting his ability to do his job. After returning from the Arctic, he tried to quit, and he got sent to The Village.
There is a link between The Village of "Colony 3" and The Village of The Prisoner.
"Colony 3" features a seemingly innocuous English-style village set up in an unnamed Communist nation. Even though it's purpose is explained early in the episode, and lacks the surrealist touches of Number Six's prison, they are rather reminisent of each other. Perhaps it was a test-run for the real thing?