I guess that depends on what you consider expensive... usually the steel diff is the expensive part.
I did see a price of $AU450 for this kit. Not sure what shipping from NZ to the UK would be, but all the parts I have bought from Bits4vits were not unreasonable in their shipping cost going the other way.
The guy who does this kit also made the rear disc brake one we have on our 96 Vitara. Uses Mitsubishi Magna calipers.