Hi Graham ,
I'm no expert ... but from trying to sort out my own problems I've managed to find my way around parts catalogues and from what I can see the shocks on a Cabriolet are the same as a European 92 to 98 K11 hatchback , they're the same part number . On saying that , the Euro parts catalogue is divided into different regions which are identified by a code and not by country and I havent figured out which code is for the UK.
If the forum mods allow it I would ask the same question but post it in the regular K11 forum as there are a lot of knowledgeable people in there who might not read the Cabriolet Forum - try to get a second opinion .
If you get confirmation of UK units fitting your Japanese Import then there are a lot of sellers on Ebay selling brand new shocks .
There are several K11 groups on Facebook too - you could check with them for advice too.
All I can say is that I wish I had your problem , sometimes sourcing parts for a Ford Model T would seem a whole lot easier : )
Let me know how you get on.
Best of luck !