What a nasty thread. Just because the OP isn't throwing cash around like it's going out of style, she's getting called tightarsed etc. I get the sense that posters seem to think if someone can afford a nanny then they must be loaded and should be paying the poor nanny as much as possible.
I have two dc in FT childcare in Central London. It would be cheaper for me to hire a nanny. After childcare and my travel, I am working for less than a couple hundred pounds a month after tax. Just because I can technically afford for two children to be on childcare FT doesn't mean I am rolling in it.
The nanny doesn't have to take this job you know. If the pay is so absurd, she can say nod find something more suitable.
IMO, taking away the major major expense of rent/mortgage, I would be very happy if I was earning that kind of money. If the nanny isn't happy, it's her responsibility to negotiate a better salary.
OP will start at the lowest starting salary and the nanny will or should, start at the higher end of the scale. If they're both happy, they can meet somewhere in the middle they're both happy.
I don't see why OP is getting so much abuse for not wanting to start at a very high salary. When I've negotiated my salary in the past, I wouldn't dream of accepting a first offer.