I suspect your eldest daughter's starting salary is not in any way typical, Xenia. Extrapolating from this situation will not reflect the reality for most people.
It's true that with the low interest rates of the past few years, many people have borrowed multiples of salary that would have been unthinkable previously. It is not a genuine indicator of affordability given that very few of these loans will be at a fixed rate for a long term, and they are likely to prove unsustainable over the next few years.
You get a much better idea of affordability if you look at the numbers of first time buyers, and in particular, unassisted first time buyers (ie those who don't get large deposits from their parents). All this inforamtion is on the CML website.
With regards to your "Chelsea" comment, I can assure you I am under no illusion that your average middle class professional might be able to live anywhere near SW3.
Your Harrow house does look like pretty good value compared to many in zones 3/4, but it is still well beyond the reach of most would-be first time buyers.
You really are astonishingly out of touch if you think affordability is not an issue. Even Mervyn King said, last May, "Relative to average earnings or incomes, or anything else you could look at, house prices do seem remarkably high."