It's pretty much worth it if you've played both Kingdom Hearts 2 and Chain of Memories before. Then, the Japanese language shouldn't be a problem.
The new boss battles, areas, abilities and the new Limit Form made a huge improvement over the original and offer loads of new content.
Same with Re: COM (the extra game that comes with the set, a 3D remake of Chain of Memories), the new sleights alone made it worthwhile for me, the gameplay just works a lot better here.
However, since you live in the UK and probably have a PAL PS2 system, you should be aware that some problems can occur with using Swap Magic on PAL PS2s:
- Resized screen, black margin at the bottom (just about 10% of the screens unused)
- Video and Sound are often off-synch in normal cutscenes (isn't that big of a problem though), sound barely works at all in animated scenes like the secret movie, opening, ending, etc.
- The game is slightly slower than the original (but really just a bit, most people don't even notice it)
I think these problems only occur if you play it on an old 50hz TV, which is not compatible with the 60hz NTSC format, so if you have a 60hz TV, it should run fine.
If you use a 50hz TV, however, you probably should just buy this game for the gameplay, and not for the story and cutscenes.
Oh, and if Play-Asia.com doesn't work out, I guess looking at Ebay UK should do the trick, too.