I knew someone who had once been an RPO. If his stories were to be believed, even the late Queen used to go into Windsor sometimes and be able to shop without being recognised. He said it was a nightmare but she still did it!
I was once in Waitrose in Windsor (which is practically in sight of the Castle). At the greeting card section there was a woman with her back turned to me, muttering to herself. I could hear that she was very well spoken, she had a headscarf tied around her hair in the manner that the Queen often did, and she had a tweed type skirt on.
I was stood there convincing myself it was indeed the Queen, and wondering if I could bring myself to ask if she needed help choosing a card, when she turned around - and of course it wasn’t her at all!