> That doesn't change the raw GDP figures though

That's assuming they're relevant, and spoiler, they aren't for the topic at hand.

How is any individual's life better because they work at a café selling $20 coffee vs £5 coffee? Because if you measure GDP, person one is drastically more productive, but in reality, it doesn't matter, really. If both are earning minimum wage, the person in the UK with the cheaper coffee has higher income (outside of a few US states). But what is their quality of life? That's highly dependent on where they live, what are their housing costs (which are quite high in the UK in most desirable locations), etc. In any case, GDP doesn't evaluate that, and it's probably the main thing people care about and what they mean by "poor".

The point is that it wasn’t that way in 2008, and in comparison the UK has stagnated and been overtaken since then.

I agree, GDP is an incomplete measure, but it does provide a comparison point.