We have come across more than one teenage who relies on the tutor to do homework. Indeed, and this was a boy at a very famous school (not W), the tutor did the GCSE coursework. My children tell me they learn best by listening in class, which is a good learning habit. Some kids instead learn to rely on tutors instead to go through material, which must frustrate teachers
God forbid! not W but the other famous school, not my children but the other children.... Very funny 
OP sounds a couple of mothers I know, indeed I wonder if I might know her..... Its been strange. I know there is an element of this in MN but in the world of central London private schools, there seem to be ranks of hugely gifted children. DC never were. At times it has felt as if the parents have judged us on our ordinary public sector jobs and then their children have assumed the same attitude towards our children. (A bit tedious, and not dissimilar to the attitude towards career tutors which seems to exist here.)
I don't live in London but I spend a lot of time in London and I know a lot of parents from W and SP who tutor their DC in hiding.
What makes you think we are not in the public sector as well? What makes you think that my DH doesn't work for the Foreign Office, for example? Stop projecting your biased assumptions motivated by misinformed prejudice.
Have a good sleep Needmoresleep 