It has very little to do with the “competitive vigilance of the press”. As MSP says, it has mostly to do with legalisation. In cases where corruption within police forces has been exposed, such as in miscarriages of justice, then often it is the result of tireless campaigning by individual MPs backed up by investigative journalism by TV productions such as Panorama.
For example the West Midlands Serious Crime Squad was eventually disbanded as a result of campaigning by Chris Mullin MP and Granada Television documentaries.
Corrupt politicians such as Jonathan Aitken tend to be exposed by the investigative journalism of newspapers such as the Guardian.
The motivating factor in these examples is not the “competitive” vigilance. That it left to the gutter-raking press as the dredge for celebrity “drugs and sex scandals” …….. not corruption.
