Nannies salaries do not generally vary according to the number of children, thus nannies are a lot better cost wise when a family has 3 or more children. With 2 children the cost difference can be negligible, but a nanny for 1 child is very costly. Just my view of course
If you want to have a comparison, as a nanny I care for 4 children, and it costs (including tax, NI, employers NI, weekly kitty etc.), less than £10 per hour - thus under £2.50 per child, per hour. A childminder in my area charges typically £4-£5 per hour, per child... so a nanny is better value for families with more children.
You will find that nursery costs go down a little (and it can be very little) when children reach certain age points - it depends on how the nursery does their pricing. Once in the term following their 3rd Birthday, government inject a little money for nursery education, so cost drops then, though not by a huge amount (should be noted, that nurseries are not the only childcare provider type which gets this government money, some childminders also provide Nursery Education, nannies/home childcarer can also provide it - but to my knowledge, government won't pay towards that!).