it was explained to me by a fiend who is a nanny, that the reason a nanny's hourly rate is so high is because when paid like that she is self employed and responsible for her own tax insurance etc.
DP used to be a payroll manager for a company that specialises in payroll for domestic staff, mostly nannies.
The convention is, apparently, that the net pay figure is what's guaranteed, so if they quote £24k, the nanny would get £2k pcm and the employer would be responsible for tax and NI on top of that. (This has the interesting upshot that if, for example, a nanny has deductions, eg student loan, the employer is paying that, as it's a net pay contract).
I'd be surprised if a nannying job, where they are working for just one family at a time, could be regarded as being truly self-employed. Some of the tests in IR35, such as having to do what the "client" requests (within reason), sending someone else to carry out the nanny's duties and the client having control over where and when the "contractor" does the work would surely make the relationship an employer/employee one?