I have to diasagree with the poster who said it is her choice as she is an adult. Quite simply she is living in your house and therefore your rules apply, as long as they are within reason- and sticking to national guidelines definitely is.
Last lockdown we asked our nanny to move in with us as it seemed the only viable choice. We were very clear that she had to follow the national guidelines or it would result in her employment termination. Luckily for us she was fine with it. Admittedly it’s easier for her- a woman in her 40s, than it must be for younger people who just want to have fun. I feel for them, I really do, but not enough to change my mind!