The requirements for Mountain Lion and El Capitan are: Your only other upgrade is Mountain Lion (10.8) which is available in the USA (but not the UK) in the Apple Store:
Note that if your Mac is capable of handling Sierra this link is not available to you. If it isn't, but is capable of handling El Capitan you can upgrade to that using this special link:
If your Mac is capable of handling Sierra you can upgrade to that for free.
I'm afraid Mavericks is no longer available.
