In Aus, the two things that retained inch measurements for a long time were personal height, and bookshelves (I guess that says something about the people who buy bookshelves …). But a lot of things are still implicitly the old measurements: I buy timber at standard 3’, 4’ or 6’ lengths, it’s just now described in millimeters. I don’t know what a polytunnel is, but I would expect the actual length to be 6ft, regardless of how it is described.