How to upgrade to Mojave/Catalina from Sierra in 2021?

Hi,

I have a late 2012 iMac that I've recently pulled out of storage and would like to update to Catalina (the most recent OS it is officially capable of running). I have spent hours trying to find a way to do this, but it seems impossible. No OS earlier than Big Sur is listed in the App Store, and attempting to access Catalina/Mojave/High Sierra links found on these community boards just opens iTunes with an error message stating that the item is not available in the UK store (I've tried Safari as well as the App Store, with the same result). Nothing I try leads me to what I'm looking for. How can I access Mojave or Catalina in 2021? Or even High Sierra for that matter? Any help appreciated - thanks.