My MacBook Air (13-inch, Mid 2011, bought and running in UK) is running Sierra, and I was trying to upgrade to Mojave but apparently it's not supported anymore. I'd like to upgrade to High Sierra instead but it doesn't seem to be available in the App Store. i.e. I search for High Sierra or any other OS names and find no results. Is there any way to do this?
Any help much appreciated
(p.s. I tried asking this on the apple discussion board but I can't seem to post anything - "You are not allowed to create or update this content")