I always thought I wouldn't mind cameras and assumed I had them in one house (not UK) and never a problem.
Then in another job there were secret cameras. First became suspicious when the mum would call/leave notes about something I just happened to have been doing at the time/that day e.g. cooking the wrong way (with oil not water), giving a whole avacado instead of half, giving juice alongside a meal that included potato, said she was having 'funny dreams' about this that and the other (even had them in bathroom turned out).
Housekeeer eventually confirmed it. Mb used to play us off each other and try to cause trouble/give reason to tick one of us off to see if we would 'tell' on the other about e.g. throwing an empty dirty bottle away that she had wanted to keep.
I felt every move was watched intensely the whole time I was at work (even if maybe I wasn't) but was never told they were there. I had to just play along with these coinicidences like/in case it wasn't true (some mght have been coincidences). The poor child began to feature less and less in my priorities. It began to be about the way I would wipe a surface etc. or if I'd have my mask on when I had cough, or if the child had socks off.
It got really stressful, they were (I felt) totally abused and tbh think I am still recovering from it. Often think of becoming CM as keep feeling I can never trust there's no cameras/parents, even if they say there aren't any.
My stratergy now is to say if there are cameras I want it in writing and for a specified time period only. Maybe She could suggest a time period e.g. two months.
Are nanny cams really illegal though if nanny doesn't know? It is not in public place.