To add on to this because the Wikipedia page doesn't actually explain it at all for some reason:
Nominet Ltd was in charge of all .uk top level domain registrations, and they simply decided that they wouldn't allow anybody to register with a raw .uk domain. As far as I can tell, they allowed .co.uk, .org.uk, .me.uk, and other such things according to what the websites claimed purposes were going to be. In 2014 they changed their minds and decided anyone can apply for the raw .uk top level domain, and now newer websites can just be called shitcum.uk
I can imagine a few reasons why huge websites like Google and Amazon don't switch their URLs to google.uk just from a business/corporate perspective. It's probably seen as a lot of money and man-hours to register the new domains, redirect their .co.uk to the new .uk domain (for how long do you even want to pay for both domains?), and the headaches of janky issues arising from the changed domain like possibly third-party APIs breaking or Boomers bookmarks no longer working.
I'm surprised google hasn't bought google.uk and redirect to google.co.uk as it's a common practice, such as wikipedia.com redirecting to wikipedia.org
Maybe it's cost, but I'm not convinced.
Kinda relevant, I recently discovered that sleazyjet.com redirects to easyJet.com lol. They actually pay for that domain
I can also see them keeping .co.uk out of legitimacy instead of switching. Like you can have just .uk nowadays but everyone is so used to the .co that it looks weird and unofficial if it's not there.
IIRC, one of the historical concerns was that some cheeky Brit would register a domain name that ended with the letter F, like buttf.uk. So “co.uk” became the awkward solution.
Thanks to Britain, we now have swearing on the Internet.
suckmy.co.uk
I remember read something about being islands, that never made sense to me, but .co.jp it's also pretty popular.
retiolus@lemmy.cat to 
To add on to this because the Wikipedia page doesn't actually explain it at all for some reason:
Nominet Ltd was in charge of all .uk top level domain registrations, and they simply decided that they wouldn't allow anybody to register with a raw .uk domain. As far as I can tell, they allowed .co.uk, .org.uk, .me.uk, and other such things according to what the websites claimed purposes were going to be. In 2014 they changed their minds and decided anyone can apply for the raw .uk top level domain, and now newer websites can just be called shitcum.uk
I can imagine a few reasons why huge websites like Google and Amazon don't switch their URLs to google.uk just from a business/corporate perspective. It's probably seen as a lot of money and man-hours to register the new domains, redirect their .co.uk to the new .uk domain (for how long do you even want to pay for both domains?), and the headaches of janky issues arising from the changed domain like possibly third-party APIs breaking or Boomers bookmarks no longer working.
I'm surprised google hasn't bought google.uk and redirect to google.co.uk as it's a common practice, such as wikipedia.com redirecting to wikipedia.org
Maybe it's cost, but I'm not convinced.
Kinda relevant, I recently discovered that sleazyjet.com redirects to easyJet.com lol. They actually pay for that domain
I can also see them keeping .co.uk out of legitimacy instead of switching. Like you can have just .uk nowadays but everyone is so used to the .co that it looks weird and unofficial if it's not there.
IIRC, one of the historical concerns was that some cheeky Brit would register a domain name that ended with the letter F, like buttf.uk. So “co.uk” became the awkward solution.
Thanks to Britain, we now have swearing on the Internet.
suckmy.co.uk
I remember read something about being islands, that never made sense to me, but .co.jp it's also pretty popular.
Also .co.nz