A car manufacturer never puts an item on a car in production that he doesn't have to. If you can save a few yen per unit, it equates to a big profit down the line when you have produced a million cars. Hence, where constant speed cruising at 70mph (or more) is the norm, a bigger oil cooler might be necessary, but in Japan where sustained high speed is relatively uncommon (allegedly), the smaller oil cooler is sufficient. Same with the transmission cooler for the auto - the UK has two, one in each radiator tank! Also, differential coolers may be added for European markets.
The U.K. oil cooler is a straightforward swap for the JDM version, but needs some support as it is taller and will wobble about at speed (UK cooler brace can be used). Aftermarket coolers, e.g. Mocal, are more efficient shorter units and could be fitted with minor mods and no brace. Be careful not to overcool the oil - does anyone know if there is thermostatic control over oil flow to the cooler? If not, then an oilstat in line would be advisable.