One of the reasons that, as is often said of trying to keep a Jag running in the US, "All the parts that fall off are of the finest British manufacture."
And there are lots of WWII stories about trying to fix Sherman tanks and Jeeps with Whitworth fasteners.
Actually, UNC is Unified National Coarse, pretty much the same as the old USA standard which was SAE (Society of Automotive Engineers). The difference is in the small radius at the bottom of the V. I can't remember what the change was for sure. I think the USA thread had a sharp V at the bottom of the thread, and the Unified National Thread (adopted by US, Canada, and I'm not sure who else) has a small radius. The old SAE threads will fit just fine with the newer Unified thread.