Ah. Those government stats only count homes as having loft insulation if they have more than 125mm of it (this is actually documented in the PDF you linked but

Ah. Those government stats only count homes as having loft insulation if they have more than 125mm of it (this is actually documented in the PDF you linked but it's all the way on page 24 so if you just read the summary you probably missed that). In practice there are a lot of UK homes out there with loft insulation that falls short of that because there's rapidly diminishing returns from adding more and it's a lot of extra hassle to put insulation in that's deeper than the joists. Those homes have much, much worse return on investment on adding loft insulation than a house which genuinely didn't have any.