According to this page is yours a late 2008 Aluminium? If so you are good to go. If it's white polycarb, I'll say no.
Being in the UK is not an issue.
You'll need to purchase 10.6.6 from a reseller or Apple before you can get to 10.8
My usual regimen for system upgrades:
- You haven't told us anything about your Mac. The first thing you need to do is make sure that your Mac can run Mountain Lion. Apple's system requirements are here --> http://www.apple.com/osx/specs/ . To find out what your Mac specs are, click the Apple menu and select "About My Mac". 2GB is the, minimum, RAM requirement for Mountain Lion. You may find it benificial to have more if you computer can support it.
- Check hardware and software compatibility. The big thing to know about Mountain Lion is that Power PC applications will not work (Apple dropped all Power PC support with Lion (10.7.x)). As you have Leopard (10.5.8.), odds are good that you may have Power PC apps on your system that you'd, either, have to replace or upgrade. To check if you Have Power PC apps, upen "System Profiler" from your "Utilities" folder. Look for Software --> Applications in the sidebar, then, sort by "Kind". Anything designated as "Intel" or "Universal" should, in general, work in Mountain Lion. Anything "Power PC", won't. RoaringApps is a fine resource you can use for checking your software compatibilty. Update software as necessary. Make sure your peripherals (printers, scanners, etc) are compatible. Update drivers as required.
- Run Disk Utility ("Utilities" folder). Run "Verify Disk" to validate that there are no problems with your boot disk. Repair any problems that are detected (you'll need to run Disk Utility from your Leopard (10.5.x) install DVD to use the "Repair" function). Run "Repair Permissions".
- Back Up. This is critical. If you follow the preceding steps, your upgrade should go without a hitch. But you should always be prepared in case the worst happens. If you're not using "Time Machine", I recommend doing so. My preference is to have two back-ups. I keep incremental back-ups with Time Machine and maintain a bootable clone. Carbon Copy Cloner is my preferred application for cloning (for Leopard, you'd need the older version of CCC -- 3.4.7 -- still available on the site and still free). SuperDuper is, also, highly recommended for clones.
Upgrade away and good luck!