Welcome to the modern economy. This is not uncommon or suprising. This probably has more to do with production capacity in each country for various platforms or components. You have to remember that Toyota sells this platform all over the world. It could be that UK happens to be a good source for that particular powerplant, yet Japan is the best factory to assemble that particular platform. You just happened to live in the UK. That same car might be sold all over the world.

A complete engine assembly is a high value asset for it's size and weight... so shipping it a long distance is not unreasonable. Having the engine made in 5 or 10 different locations to save a few dollars on shipping costs would be rediculous.

Also consider that the engine makes up a large percentage of content. Therefore, the tariff would be lower I beleive if the UK content is higher. I don;t know if there is a assembly plant in the UK (I thnk Honda or Toyota are building some in Ireland), but if there is, it also possible that capacity for that platofrm is maxed out there, yet the UK is hte only source for that motor.