No the point does not stand at all. £42 000 puts you into the well off bracket wherever you live.
I’m afraid this argument does not stand up at all…
42k in part of the country where accomodation costs are measured in hundreds, then yes, quite likely that person will feel comfortably off.
42k in an area with very high accomodation costs – then no, not well off.
My BiL left the UK for the US as he could not afford to support his family. He was above the 40% threshold, and his wife was working parttime around childcare / nursery hours etc.
The industry he works in coalesced around SE England, and he was forced to move there with his job. He could not afford to buy, and private rents on family homes were stupid money.
So yes, his earnings were top 10%, but his disposable income was very low.