Your software IS up to date for the OS that you bought or which came with your mac.
If you would like to UPGRADE to 10.8, you need to purchase that through the App Store ($20, I think).
10.9 is not released yet, but is due around Fall/Autumn. It will also be a paid upgrade. It's not clear yet whether you can upgrade directly from 10.7 to 10.9 or whether you will have to go to 10.8 first.
Wowzah. Not directed at you, but, like ouch!
£15 to have a minor OS refresh and bug fix, from a corporation with $$$ in cash in the bank, seems a bit tight to me. Talk about making Microsoft look generous.. sheesh.
I run 10.8 on my MacBook pro and need 10.8 to run the xcode 5 dev tools on my work mac... I thought it would be a simple download, didn't realise we'd have to pay? Frankly the differences are in no way deserving of any price tag!
Apple - try being more generous, people might like you more instead of just resenting every time we have to deal with you. The only fun bit seems to be walking out the door with the shiny new thing.
If your MBP came with 10.8, then that license is machine specific only and can't be used on other machines. However, if you purchased 10.8 at the app store, you can download it again and use it on as many Macs as you own and control. Please note that that license is not transferable - it is tied to your Apple ID forever; if you sell the machine, you need to wipe the drive and reinstall the original OS or the buyer will be running an illegal copy and won't be able to reinstall.
FWIW, updates (10.7.3 to 10.7.5, etc) are always free; a new OS (from 10.7 to 10.8 for instance) is now about $20 (rather than the $100+ it used to be). I believe Windows users had to pay for Windows 8?