I can't comment on Kyozou specifically, but I am always careful when asking developers if they support a country because there is a lot more to it than currency symbols and timezones. The following all vary between international eBay sites:
Dates as well as timezones and currency
Sales taxes / VAT
Spelling and language
If a developer hasn't considered all these factors I don't think they can really claim to "support" a country.
Can you confirm what restrictions are imposed by your software for UK users posting to ebay UK?
Surely if its not compatible with ebay UK and doesn't include everything needed such as Royal Mail shipping options etc he had the chance to say so in his reply? I can't believe he would just ignore these points and say yes you can post to ebay uk and yes you can post in £'s.
I take it that if it can post to ebay UK then it has to use UK categories otherwise the posting would fail.
I'd look into it further but reading some of the other reviews about it on here I don't think I will bother. I'll stick with MW's for the time being. (can't stand it personally, but CA Merchant is too expensive lol)