I buy the rust argument up to a point, but it does rain in a lot of places. The RHD is of more value than a LHD in countries like the UK because it is more practical- we drive on the left. I think the more pertinent question is why are (classic) cars in the UK more expensive than their continental or American equivalents. Taxes, import duties may play a part in some jurisdictions. But unless we think the Indian subcontinent are going to become massive importers of classics, I can't see the reason.
Is there an argument that there is a greater affinity to classics in RHD countries? Hmmm. Since post war car production in the UK was geared towards the export market maybe there is a shortage of classics compared to other economies (witness the huge amount of rhd conversions AH, Jags, etc) Provenance/history is an issue here. Home markets should command the highest price for the best examples, but this is not the case with MB cars as they are sought the world over. So the provenance of a UK RHD Benz will add some value (in the UK)
Was the UK MOT more stringent than other vehicle tests thus reducing supply? Maybe, but not anymore.
I think Andy's observation is correct, but I wouldn't bank on the differential remaining static. Finally, is the huge number of UK classic dealers lead to an enthusiasts market where prices are also inflated?
HNY
Drew