Agree it shouldn’t matter - class is a social construct to help some people feel superior to others.
However, there’s still a lot of class prejudice. Over the years, I’ve had shocking assumptions made about me, and things said to me, because I’ve been judged as working class by people who’ve perceived themselves as middle class.
So, like other discrimination, we shouldn’t pretend classism doesn’t exist, or doesn’t matter, just because we think it shouldn’t happen. And it helps to measure it to identify and address discrimination.
However, it’s difficult to define and not usually based on income or wealth. Even wealthy working class people still face prejudice. A typical definition is whether your parents went to university. But that’s imperfect, e.g. it became easier for working class people to access higher education from 1960s and higher educated people can still be judged as ‘working class’.
OP: your income appears to be in a high decile. But, as others have said, it’s not only income after taxes (including council tax) but after essential expenditure (housing costs, energy costs, food etc.). People on high incomes who have to pay high private rents, high energy bills, care costs etc have much less disposable income than people on much lower incomes with inherited wealth/mortgage-free.
Also depends on how many people are in the household and whether they’re earners or non-earners. That’s why even above-average-earning lone parents can struggle to afford essentials, like rent, energy and food.