OK - but how is ruling 1/4 of the globe down to virtually just these islands, having a virtual monopoly on certain minerals and cash crops down to being a net importer from other powers not an economic collapse?
Is it really an ongoing collapse in perpetuity?

Britain's population growth is negative without immigration - it is not healthy.
According to your ONS, the birth rate is rising - though admittedly that is partly a consequence of immigration, most of the risebeing attributable to immigrant mothers.

In terms of education, military strength, health, and economics we're not making positive growth over the long-term.
Economically, you're richer - period. Technologically, clearly stronger. Health? So good in some aspects that problems in others have been engendered. Militarily, you're still relatively powerful - isn't relative power what you're always on about?

They're also symptomatic of an acknowledged cultural malaise that makes people not want to procreate.
In Japan? I don't know...