My bf is a retired royal protection officer. Most of the comments above are accurate. Indeed, it falls under the Metropolitan Police. There is a highly trained branch that oversees royalty and diplomats, and within that, some police are uniformed and some are not. On occasion, if there is a direct threat, a regular private citizen may get protection for a time. For example, my bf had to guard Salmon Rushdie (who was a horrible, nasty person, according to him. The British taxpayer pays for this, and despite him making millions from his books, he never repaid a penny for his security). If Harry and Meghan come to see the Queen or Charles, they will automatically be in the Met protection bubble, and that goes if they're on the Windsor Royal Estate, which also covers Andrew and Sarah since they live on the estate. Now if Harry and Meghan want to take a stroll around the town of Windsor, they're on their own, as that jurisdiction falls under Thames Valley Police. Official occasions, like the Queen's Jubilee, they would be protected. However, on casual visits, they simply cannot expect to be provided with or rent their own Metropolitan royalty protection officers. And from what I've heard, I don't think any protection officers would be lining up for the extra money--even if they could.