The Prisoner may have resigned from his job, but who is to say that resignation was accepted? After all in his former life the Prisoner's work was of national importance, top secret and of a confidential nature. One does not simply allow a man like that to walk away. Something has to be done about him, and quick!
It's always possible that the Prisoner had become suspect, seeing as he was under the closest possible surveillance. "What was that? Sounded like a click, something in the mirror? Or was it over there? Yes over there too! Well there wasn't actually anything in the mirror, just behind it in fact. Something rather like this.
If you think it was inconvenient for the Prisoner to be under surveillance, spare a though for his next door neighbour. Who not only had had a hole in his living room wall, but also a camera set up in it!
Perhaps it was like this for Chambers late of the Foreign Office. So what to do with such men? If they had been Russian KGB agents perhaps they would have been put up against a wall and shot, other than that to one day find themselves in a Russian Gulag in Siberia. But no, the British have a much more civilised way of things. They put such people in The Village, completely isolated from the rest of the world, in order to protect and keep safe the information in the heads of people like the Prisoner. Yet that doesn't explain the fact that they extracted information from people like Cobb, Dutton, and Chambers, and attempted to do so with the Prisoner! Perhaps it's a different department of British intelligence behind The Village, and they wanted the secrets of another department which Cobb, the Prisoner, and Dutton worked for, within British Intelligence. That would explain the need for the extraction of information!
Be seeing you