I am not a mum yet but Stan I am really surprised to hear you were not made aware of this when you joined. tbh it's not something I would think to ask about either, as I would assume if the nursery had a Christian ethos they would make it clear to parents when signing up.
I'd take issue with the idea that 'this is a Christian country'. A nation is essentially an imaginary construct - it's mutable - it's a notional community made up of what people think the nation is about. Since lots of people demonstrably don't think this is a Xtian country, I don't think you can simply retort 'it is'. My experience of living here is completely non-Xtian, for e.g.
I also think it's interesting that the nursery seems only in practice to be using Xtian prayers. Despite what the manager said, I doubt she is really doing Hindu prayers on Monday, Sikh on Tuesday, a reading from the Torah on Weds, etc. So it is a de fact Christian thing, and you should have been told that.
Here is a genuine, non-arsey question to Christian parents on this thread: if your DC were at a non-religiously-affiliated nursery, and said grace most days, would you also be happy for them once a week to say something like this? -
Mrs Brown in the kitchen made this food, and we are very grateful to her, and not God, who doesn't exist - thank you Mrs Brown!
If you would not be happy with your DC saying that once a week - after all, won't hurt them, will it! - how do you justify slamming the OP?