What makes the UK have such a rich history of audio and hi-fi equipment and gear?

I would hazard a guess, it’s related to the UK having a rich history of pop music?

The UK is home to such popular acts as The Beatles, The Rolling Stones, The Who, Led Zeppelin, Pink Floyd, Jimi Hendrix, Cream, Queen, Oasis, Radiohead, Blur, The Ramones, Sex Pistols, Elton John, George Michael, David Bowie, Eric Clapton, Spice Girls, Westlife, Phil Collins, Rod Stewart etc

that’s quite an oversized amount of influential musical acts compared to the relatively small size of the UK. The UK often rivals the US when it comes to pop music influence.

And accordingly I would say the UK also rivals the US when it comes to audio / hi-fi manufacturers:

Speakers: KEF, Bowers&Wilkins, Monitor Audio, Wharfedale, Leak Audio, Creek Audio, Acoustic Energy, Neat Acoustics, PMC Audio, Rogers, Harbeth, Graham Audio, Ruark Audio, Quad, Tannoy, Spendor, ATC, REL etc

Amplifiers: Cambridge Audio, Rega, Arcam, Roksan, Naim, Linn, Cyrus, Exposure, dCS etc

That’s a rather crazy number of audio companies that are headquartered in the UK. But I think given how early the British industrialized, combined with how popular British pop acts became, it’s not surprising that many audiophiles emerged from the UK and thus you have all these audio companies emerging from the country.