To repeat a previous user's advice (thanks), the issue seems to be that the link Mozilla provides links to the below URL but with "en-US" where "en-GB" is. If you change the address, it works fine.

addons.mozilla.org/en-GB/firefox/addon/english-gb-language-pack/

How Mozilla hasn't picked up on this for so long is beyond me. If the technical cause of the issue is what I suspect it is, it'd be a straightforward fix.

Love these language packs, I have different versions for English, as well as my mother tongue french.I also have my other languages that I am slowly learning. Our team we have another service that we love but honestly this tool is a must when working with Mozilla Firefox : it never fails me.