Multiple causes
Cost of living, accommodation in big cities in particular. ZIRP had a lot to do with this - rents rose in line with values.
Working hours and conditions that are not family friendly.
Pressure on couples with kids to both work full time, which then means a lot of money goes on childcare so they are not that much better off (but landlords and banks are happier with the lower earnings multiple anyway) - but it boosts GDP and profits so that is fine.
A benefits system that reduces payments too rapidly when people earn. It means people keep very little of what they earn. Personally I think there is a good case for UBI as the solution.
> Around 70% of children living in poverty have at least one parent in work.
That should not be happening given there is a reasonable minimum wage.
Its not a UK only problem. The article says.
> De Schutter noted that the country conformed to a pattern of increasing inequality seen in other wealthy countries.
I think a lot of it results from a shift in attitude. The people in power increasingly think poor deserve to be poor, and that they are all "gammon" (to use the British term) and untrustworthy anyway.
Your claim is that this problem is getting worse?
Nitpick: "gammon" as a constituency refers to boomers and older Gen X, typically financially comfortable, who are declining in intellectual openness and increasing in strength of opinion. They are called "gammon" because their faces go bright pink as they rail against the EU, immigrants, woke nonsense, and the laziness of today's youth. They can come from any strata of society, but they are made by being insulated from economic reality during their intellectual decline. Their defining characteristic is that they are choleric about topics of which they know nothing, and this makes them easily led by jingoistic tubthumping.