I've read somewhere that the popularity of katsu curry in Japan can be traced to the use of British-style curry powder by Gurkha troops during the post ww2 occupation, and its subsequent promulgation. No sources for this though, can someone confirm?

Well which Colonial possessions are we discussing? It very much depends on where and when we are discussing since in the 19th century the acquisitions in Africa were very different from say, New Zealand. And earlier on, in the 18th century, the way French Canada or British Caribbean possessions were treated was different still..

The free trade within the British Empire undoubtedly opened up cheaper goods, with respect to cuisine, spices from India?