After handing in his letter of resignation, the Prisoner returned to his home to collect two suitcases, his passport, and airline ticket. The question is, when did the Prisoner purchase that airline ticket? At least by the day before he resigned, which makes his act of resignation one of predetermination, and not a spur of the moment thing.
In Do Not Forsake Me Oh My Darling the Prisoner is returned to London the day before he was to have resigned his job, and sometime after he was to have attended his fiancees birthday party, that of Janet Portland, which is what was supposed to have happened had the Prisoner not been abducted to the village. But then would it? I don't think the Prisoner had any intention of attending his fiancees birthday party, simply because of what we witness in the opening sequence, together with the evidence of the Prisoner's passport and airline ticket which is compelling, together they are conclusive. He was on his way to London Airport, but whether or not the Prisoner would have called to see Janet Portland before he left is unknown. But seeing as the Prisoner was in a hurry, I would suggest that going to see his fiancee was the last thing on his mind!
Be seeing you, if I don't resign first that is!