all very plausible but I cannot help but think there are secondary issues when it comes to the performance of the aggregate economy. the UK has been so fucked in terms of macroeconomic management since 2007 that that should take centre-stage when it comes to aggregate underinvestment.

OTOH the UK has been generating similar GDP growth with a falling investment rate & thus implying K productivity has been rising. and the I rate has been falling for a long time, so maybe recent policy mistakes are secondary, and what's needed are in fact deep supply-side changes to raise the I rate

That's definitely my prior, but presumably it's some combination of spending and taxation. ie Germany has had much more "austerity" as measured by the decline in public debt; but gov spending as a fraction of GDP itself has remained more constant, while declining in the UK