Dai Davies, my old boss, has said this predicament has not arisen in the hundred years or so of Royalty Protection. I am aware that Edward V111 had a minder prior to his abdication who often bailed him out literally. Try ,as I may, I haven't managed to find evidence that he was protected in France leading up to the second World War when he fled. I cannot ascertain if he had protection in the Bahamas either. I know that he was a person of great interest to M15 due to his pro Nazi sympathies but that is not close protection.
It was Prince Harry's decision along with his wife to desert his royal duties. He or the Royal purse should pay for their security now. Obviously Harry didn't think this situation through clearly. His mother would undoubtedly be still alive had she been afforded proper police protection along with the logistics and back up that comes with it. The tax payer should no longer pay their bill.