Seems to me that most jigs are made of metric size birch ply, and that with wood, precise fitting requires a story stick rather than a tape measure. And for the most part, if your drill bits are all metric, then you can simply use the same pieces of hardware, bolts, nuts, etc you see in the jig, but metric sizes that are reasonably close to the Imperial sizes. There aren't very many jigs where the bolts and set screws being metric or imperial makes much of a difference. My brother once owned a British motor bike that used both and that it was a serious pain in the bum working with all the mixed sizes. Not sure about UK, but here the big building supply stores have drawers loaded with all the parts one would ever need in both metric and imperial sizes. Those shop notes magazines are wonderful.