I think it's the area; Central London does have better nanny jobs in general.
I have noticed less experienced foreign nannies charging more than what I thought was the going rate, and when I questioned this, the reponse was that they thought they were experienced enough...
I think the main problem is parents using au pairs as 'cheap nannies'; these families encourage these au pairs to think they are as experienced as nannies (which by and large they are not), and they further blur the line between the two.
Au pairs shouldn't look after babies, however, far too many families advertise for them to do so... I think this also leads to exploitation of these young people who come to this city. I have known families that use au pairs this way, and I have spoken with ex au pairs with rather bad experiences working in London.
My experience, and those whom I spoke with about childcare, has been largely positive. I knew what I was looking for, and quite franky, I didn't settle. I did have specific needs, however, so I think this helped ensure I had more suitable candidates.
I think if you want to find more nanny jobs, particularly full time, then it is best to specifically look for childcare websites.
If you are a family seeking a nanny, whether they be part or full time, then set some basic standards. Only consider candidates with at least 5 years experience, and at least two references, for example. The more specific you are about experience, qualifications, and pay, the less likely you are to get unsuitable reponses.