A nanny charges per family, rather than per child. A nanny's rate will depend on her qualifications and experience, not the amount of children he/she looks after. Though if you had 4+ children, you may want to up the hourly rate to make it a more attractive job!
We're in Kent (within the M25). We pay 11.20 gross for a qualified and experienced nanny for 33 hours a week, spread over 3 days a week.
Other costs to having a nanny are employers NI, annnual fee to payroll company which is about 120 or so), cost of any activities, travel (per mile if they're using their own car during the working day, running costs for your car if they're using that), food and drink for the nanny during the working day (though our nanny eats her meals, which are breakfast and lunch, at home, so not v expensive) etc.
Are you considering nursery? If you are, for comparisin purposes, if your DC were at nursery, all of their food and drinks, nappies and activities would be included in the nursery fees. Also, you would not be heating your house whilst they are at nursery, so that is maybe something to bear in mind when doing a comparison.