I've had murder getting the Ultimate Sith Edition here in the UK too. Everywhere either says unavailable or out of stock for the PlayStation 3 version. I've seen a few Xbox 360 and PC versions floating around though.
I spent days searching for a copy on UK based sites and shops, but this afternoon I just gave up and found someone on eBay selling this game in America.
There are multiple copies available, so you should be able to pick a copy up here: