Actually, I just bought MK2 Gti with air conditioning (could be the one you saw on the ad). It is imported from Australia. Also, I know the ones imported from Japan surely have aircon (otherwise people wouldn't buy as it is very humid there).
I remember someone in Japan fitted aircon by VW dealer. I am quite sure you can get some bits and pieces to fit aircon to your golf from VW dealer (maybe in Germany). However, you need to spend lots of money on that.
So maybe if you cannot find the one with aircon here, it would be easier to import from Japan or Australia. I have seen hundreds of golf2 gli and cli (which has got the same engine as gti) which cost you around 500-1000 pounds (if you spend 2 grands, you will get a perfect one). My friend told me he spent 600 pounds to import a car from japan to uk.