Ok, this will go around in circles.
The 'work' angle is something that the press has been insisting on, to confuse things. As has been said a number of times on this thread, it's a threat level that matters.
So Blair doing commercial work, has nothing to do with anything. He is a high target due to his role in the past -fair of him to pay if he takes more PO's than he is assigned for. But that does illustrate the fact that you CAN pay for special protection.
David Cameron still has his protection. Possibly Teressa May too.
Former Ambassadors - who retain their titles and certain Diplomats retain security after serving office. For all these individuals it does not matter that they are making money afterwards or if it's lucrative or not.
Some of the above mentioned may reject the security (some have cited that they are not really doing much, and the security needs you to know what you will be doing in order to do their job effectively, among other things).
What matters for Harry is his proximity to the heart of the crown. To HMQ, Charles and William (quite possibly George too). When Prince Philippe stopped his duties, his security was not withdrawn. This would have been due to his proximity to the Queen and monarch line.
I think Harry may use the examples above and will most likely win.
There is a lot in the press about ' oh but then any celebrity will then ask for private protection'. Not really, are they related to the queen? Did they have a diplomatic role in the past where they were entitled to this protection?
Also just to point out that the press keeps pretending this is Home Office and Met decision sorely. It is a decision of RAVEC - the Royal and VIP Executive Committee, so very much HMQ and the palace decision as well.