I've recruited nannies several times in the last few years, in the fringes of London, and it has varied a lot. Examples of expectations were:
20 years+ experience, qualified, English native speaker, Ofsted registered - £13 gross/h
6 years+ experience, no qualifications, non-English native speaker, Ofsted registered - £12 gross/h
2 years experience outside the UK, non-English native, no qualifications or Ofsted reg - £9 gross/h
(all sole charge, live out, not shared, 30 to 40 hours a week over 3 or 4 days)
I found that native English speakers tend to expect higher pay. Obviously, less experienced/qualified nannies expect less.
8 hour day is quite short for a nanny (usuallly it's 10 to 12 hours) so you might have to pay more per hour.
When you're trying to find out average pay in your area, bear in mind that when people say "net" pay, it doesn't necessarily mean that they will pay, or expect to be paid, tax + NI on top. Even though some jobs are advertised with a high net pay (say £12 net), it may actually be paid cash in hand (and the nanny may be happy with that). Whereas if you pay tax properly on top of £12 net, it's more like £14 or £15 gross.