Lucy, I'm sorry that you and yours have been facing these issues, but I'm glad you and DD have resolved them for now.
I understand that it's your home. However, you chose to rent it out to DD, and Ive never known a renter who didn't feel as if the flat was "their home' or a landlord who expected to be able to come and live there. So I can see why DD felt she should be able to keep using the shower she has been using for the many months you are not there.
Then again, I think it was rude for her to ask you to have OH be up and dressed when she comes in to shower. That sounds, as one poster suggested, as if she just doesn't like to see you in bed w/ a man. Very immature and unrealistic of her, IMO.
I hope the new showerhead idea works out. If not, I agree w/ the poster who suggested a curtain around your bed.
But I also agree w/ the posters who say that the real problem is the living arrangement. I understand why you let DD live there for a low rent, and I think it is very kind, generous, and loving of you. But the idea may have outlived its usefulness, especially, as you point out, if Brexit goes through. So I'm glad you're ready to talk to DD about other arrangements if it comes to that.
Also, I think it's very kind and wise of you to consider DD's new relationship. And I join you in hoping that things work out w/ her new BF and they move in somewhere together (not your home, LOL). Good luck to all of you!
Soop's warm and welcoming kitchen for kindly folk.