Why does Britain feel so poor?

Because we’re heading back to the Victorian era in terms of inequality.

https://en.wikipedia.org/wiki/Victorian_era

In the history of the United Kingdom and the British Empire, the Victorian era was the reign of Queen Victoria, from 20 June 1837 until her death on 22 January 1901.

https://ifs.org.uk/inequality/wp-content/uploads/2021/11/IFS-Deaton-Review-The-history-of-inequality-1.pdf

That doesn't look a lot like Victorian-era levels of inequality to me.

I think that the points highlighted in the article are probably stronger. For example, the author mentions housing:

Advocates of tax rises point out that actually, middle income families face lower taxes than in many other developed nations, and that’s absolute true, but it overlooks a glaring problem: housing costs. Britain’s houses are cramped, ancient and in the wrong places leaving workers with longer, more expensive commutes. And we get to pay more for the privilege, almost single-handedly reversing the relative wealth of many British families versus their European counterparts. It’s a cliché that every other policy problem in Britain resolves to a housing problem, but in this case punishing rents and mortgage payments are consuming a large percentage of the income that other countries would tax.

It's true that the UK has exceptionally small per-capita space allocated to housing compared to similarly-wealthy countries. I suppose that one could argue about whether that's a positive or negative --- I think that many people have made a valid point that people here in the US are purchasing housing that's inefficiently large, putting too much of their assets into real estate --- but it's an aspect where the UK really is unusual compared to peers.

kagis

This is just the first result that comes up. It isn't a per-capita visualization, but I think that it definitely drives home the point:

https://worldpopulationreview.com/country-rankings/house-size-by-country

goes looking for a per-capita measurement

Here we go; I've seen this visualization before.