Hey TT,
Apart from the leather you mentioned, the UK cars came with 3 seperate oil coolers. If I'm not mistaken, engine, transmission and diffs. The engine cooler also meant the front bumper design was slightly modified.
Also, I think the UK versions came with a revised MFD, although not the final one made by NISMO. Finally, the dash is slightly different as they removed any instances of the Japanese spec SatNav and CD controller.
Bear in mind the R34
does need the oil cooler especially if you plan on tracking it, so if you don't buy a UK one, make sure you retro fit a good one like I did. There are also debates as to where it should go, make your own mind about that.
I also upgraded my MFD to the NISMO II version, which is better than the UK one also. So in reality, if leather is not your cake (wasn't mine, get thrown about too much on the track) then you could spec up an import version. But beware about spending more on the long run. Although the components I've upgraded are better than the UK versions, expect to pay a little more in the long run.
Warranty is also another issue to think about, but if you buy from Middlehurst (only official UK Skyline dealer) then you will be bound in chains to them, so you can forget about getting aftermarket exhausts through after market tuners.
I've never heard of anyone using their warranty on an R34, so I can't see there being an urgent need of one.
Cem