OK, I am now in Cyprus, and I think I may have found more info to help uncover the mystery. A bit, anyway.
Since arriving in Cyprus, I have seen two more cars with this exact same spoiler, and have had a chance to talk to the owner of one of the cars. And I think I see a pattern emerging. All of the cars I've seen with this spoiler are Version IV STi's, not versions V or VI. And rumour has it that this is how they came off the boat from Japan. So, not a UK version spoiler after all. It is entirely possible that the shipper in Japan has added the spoiler to help them sell, or to try and pass the cars off as newer then they really are to unwary buyers/dealers.
As for the quality, the couple I've seen up close seem OK. Of course, I wasn't able to do a full stress test analysis, but they seemed to fit well enough. So, I'm not yet willing to dub them "crappy knock-off", but they certainly do seem to be some kind of knock-off after all.
Bye for now,
Stuart.