British GDP per capita has been flat since 2008 while the US’ has gone from $49K to $81K. The UK has declined in relative terms compared to what used to be a near peer.
It may also be true that there's a national decline, I'm just saying to be very cautious about getting swept up in the idea. For instance, apparently America needs to be Made Great Again, facts notwithstanding.
Certainly one should take TFA with a pinch of salt rather than at face value. It is incredibly partisan.