Another nanny employer's perspective. If I want to have a temporary nanny or a part-time nanny, it probably makes sense to discuss rate per hour net, eg £8 ph (pretty usual rate around here).
However for a full time permanent job in other professions, you don't normally talk in terms of hourly rates, but instead in terms of annual salaries. Just as a guide, £8 per hour net grosses up at approx £27k including tax and NI. (Employers have to pay another £1.5k on top of this as a special NI contribution, by the way). An employer needs to earn about £42k before tax to cover this kind of outlay, and tbh not many people make that kind of money, particularly in the present climate. So it's a unrealistic thing to do IMO.
I think the reason behind this way of working out salaries is that inexperienced nannies often base their calculations on flawed reasoning. They take the minimum wage and add a bit on in their heads 'because they're worth it', forgetting that the minimum wage is before tax and net wages are after tax, leading to rates of £12+ an hour. They then roll this up by multiplying it by 50 hours a week, coming to a figure approaching that of an experienced graduate teacher aged 28 reaching the top of the pay scale. Nice if you can get it at age 18 or 20 or whatever, but possibly not always realistic.
The younger nannies who get worthwhile and sustainable jobs calculate things a bit differently IME. They seem to peg their earning expectations to annual rates for nursery pay and add a bit on to that. This results in an annual salary of between about £14k-£20k + which looks a lot more reasonable to employers.
Obviously nannies with specialist skills or bigger responsibilities living in expensive parts of the country might be able to command a higher rate, but a nanny is never going to earn the same as someone at the top of other professions, such as teaching, social work or nursing, simply because the number of jobs paying £500-£750 a week net are very very limited indeed, and often involve overseas travel (not always practical or possible).
The moral of the story is of course that salaries really ought to be agreed annually and gross if nannying is ever to achieve the respect it truly deserves amongst the professions.