We have had more than our fair share of battles with local planners and conservation officers over the years.
There is not much wrong with the regulations and guidance, some of which has been modified in recent years. The problem mainly arises from bizarre interpretation of the regulations by some planners, and hopeless inconsistency. It's the inconsistent decisions which lead to people believing that corruption is involved.
While I don't condone builders/developers who build something that has not been permitted, it is often the best solution if you want to bring the planners to their senses. Enforcement action is costly so they have to be very certain of winning before they will go down that route. So a retrospective application will often pass when an application made before building would not.
Discussing this last week with a local builder, we both agreed that it is usually worth including something in the plans that you know the planners don't like. So he often puts in applications for dormers on jobs where all that is wanted is a Velux rooflight. The planners, happy that they have "won" on the dormer, let other things go.
There are places where "garden-grabbing" is acceptable. With a suitable new access road, why not create infill housing where neighbours with large gardens are agreed about it. But that is quite a different matter from the sort of shed that Horsetan is having to put up with in his neighbours' gardens. LB Barnet should be threatened with legal action if they do not act in accordance with the law and enforce against these people.
Finally, I do have evidence that corruption occurs. But it is rarely of the direct "used fivers in a brown envelope" kind. It is more about favours and influence.