I have found that given recently improved accessibility of the auctions via the dozens of agents, the prices achieved in Japan have become much more representative of the value on a global level. In the case of the Lupo, as they are all RHD in Japan, the only other market with real buying power is the UK, i.e. the price at auction + fees + shipping + clearance etc. usually reflects the perceived resale value fairly well!
While I would agree with your statement that like-for-like (if that exists) a UK car would outprice a Japanese car, the reality seems to be that overall the average quality of a Japanese GTI will beat that of a UK one, for reasons already mentioned in the thread. So yes I suppose one could take advantage of a discount with a Japanese car, hence my argument would be to work back from what the value of a very good 38k miler in the UK would be; at least that's how I came up with my number (but perhaps I am wrong!).
As a final casing point: the second car, the 43k miler I have inbound would be available at roughly the same price as then you have referenced above, highlighting the benefit over an inferior condition UK car with 10k more miles.
I am fortunate enough to be able to acquire (mostly) directly avoiding the agents and thought unmolested low-mileage examples might be of interest to this community, so again I am open for offers on the two cars!