Location may make some difference, though it's probably a combination of location and the persons experience that makes the salary difference, as even in lower cost areas to live I would imagine that experienced nannies will want good pay.
As a nanny about 25 miles outside of London, over the years I have had salaries from £7 to £10.50 per hour Gross.
The number of children you have I don't feel makes a difference... as a nanny I've worked for families with 2, 3, 4 children. The more children they had didn't mean more pay. It was far more to do with my experience level and also with what the parents could afford to pay.
Whilst nannies may want a certain income, if they can't get that then would they not take a job at all? Some hold out for the salary they desire, whereas others will feel an income is better than none, so will accept something lower with the aim of moving on to something better later.
A calculation for a 3 day a week nanny - this will give you some idea of the costs involved, such as day-to-day expenses, travel whilst on duty, payroll. That example works out as over £20,000 for 30 hours a week (it is based on a salary of £10 gross per hour).
You are looking at 40, 50 hours, so the salary you agree is the important thing, as if you agreed £7 gross per hour, then that's going to cost a lot less then £10 gross per hour.
MrAnchovy's PAYE Calculator will let you play with financial figures. You enter in the number of days per week, number of hours, then complete either the Gross per hour, Net per Hour, Gross or Net per Month, Gross or Net per year. So you could see how a certain hourly rate multiplies out as an annual salary, plus what the Employers NI (the tax you pay as the employer) is on that salary.
Use websites like NannyJob.co.uk Childcare.co.uk Gumtree.com to see if you can find any adverts for nannies which give details of salary for the area in which you are considering buying a house. That may help to show market rates, though there will be people prepared to work for less, they just may be hard to find.
As Holy says, the job has to be appealing to the sort of nanny you would want to employ. If you lived in a small village miles from anywhere... where would the nanny live? Probably not in the village as it probably costs too much. I am assuming you are looking at a Live-Out nanny... so you need to look at things like where would the nanny most likely live, how far would someone be prepared to travel. You are already looking at house prices, so also look at prices and rental costs of Studio Flats / 1 Bed Flats. Could a nanny afford to live within say 10 miles of where you would be living? On the estate I live on, a 1 bed maisonette is £135-£140k, Studio flat rental £550pcm. I live in Surrey close to the Berkshire/North Hampshire border - an expensive area but an area where there are nanny jobs.