I don't know other careers which do this.
It is wrong because
a) it sounds like tax and NI are nice optional extras for the employer not a legal requirement
B) it makes the pay sound so much less than it is. The average nanny salary in London as far as i can tell is £30k per year, which is more than a teacher starting salary in London.
I am always coscious at work how much someone is paying for my effort, and I am aware of it as the actual cost. A nanny should be conscious that their employer pays £14/hour for their help, not £10/ hour.
It shouldn't make a difference to the quality of care or anything but may help improve the perceived status of the role.
I really just don't get it, as the nannies seem to be the ones perpetuating this. It's not like the employer actually thinks the nanny is cheaper than they are, since it's their bank account the full cost leaves each month!