Here's a crude method of telling which is which:
Jap cars have smaller and squarer number plate receptacles with the various stickers in the engine, body and doors etc in Japanese. Some Jap cars, I believe, have a small "JPN" marking on the speedo (I saw this on a Jap-spec X5). Jap cars's radios can only tune up to 90kHz. Jap cars use metric units for the speedo, odo and on-board computer.
UK cars use imperial units for the speedo, odo and on-board computer and, where applicable, for the various stickers found in the engine bay and perhaps for the tyre pressure sticker pasted on the driver's side B-pillar. You might also find some rotting fish and chips in the glovebox - just kidding!
Singapore cars use metric units for the speedo, odo and on-board computer.