The tldr of the article is basically that Turing had been brought up among powerful men for whom things are permitted that are out of reach to the commoners. The mistake he made was to assume everyone else was treated the same.
People in power would not tolerate a police trying to manipulate them. By definition, people in power would have the power to change that.
Power is what decides what police does, who they target and who they leave alone. It controls the budget, appoints the leaders and sets the directions.
Power, in 1950s England, came to people from Eton and King's college. They didn't need to fear the police because they (and their family) controlled it.