It depends on where you and what sort of experience you want!
London live out nannies, depending on experience earn between £7 - £15 gross, some get even more!
Further north I would say NMW to £12 gross.
She would have to factor in employers national insurance, transport costs for nanny and child/ren, either bus, petrol for nanny's car or providing a car (plus fuel).
Depending on how many hours she needs the nanny, she would need to provide meals, drinks and snacks (I get 3 meals in my contract as I work 12 hours a day.)
Heating - the nanny will be in during the day with her child and may need the heating on, where it would not be on if the child was in a nursery/CM.
Wear and tear on the house, I have been a nanny for 28 years, I have flooded a kitchen twice, crashed 5 cars (only one was my fault and I offered to pay the excess but my employers would not hear of it) broken various cups, plates (including a christening present!) and glasses, a vase, a ceramic crock pot, a window (playing football with my charges!) and most recently a sink! But I'm sure there's more!
Activities for the child/ren - classes, toddler groups, outings to zoos/farms/soft play. Craft supplies (nursery/CM would supply these). Baking supplies. Educational toys.
It's not just the wages!