I'm umming and erring on this one — "irrelevant" feels much too strong (even NK isn't "irrelevant"), but OTOH "becoming" implies a direction of travel rather than a state, and every impression I have is that the UK doesn't matter as much as it used to.

This has been true since the early 1900s, so one’d need to show a markèd increase in the rate of derelervance

Yeah, that's fair.