>>> the cost of getting by — long ago went past insane (£17,040: the cost per year of educating a four year-old child at Thomas’s school in Fulham, not including uniform).

Note that this is only if you send your child to private school, not merely "getting by". Even among the middle classes, this is not very common, especially at four years old. The nicer parts of London have some of the best state schools in the country.