While it's possible to special order Imperial thread standard spanners, it's unlikely you are buying those. US Customary screw specifications are a separate thing, (have been so for over 100 years now), and have nothing to do with Imperial anything. There are some sizes that will fit, much as some metric wrench sizes will also fit US Customary and vice versa. But there are bunches of Imperial sizes that have no correlation to US Customary standards for machine screws.

Source: Old, cranky, retired toolmaker that has had to custom make screws for everything from laboratory equipment, cameras, to antique cars and tractors to 200 year old firearms. I'm familiar with more screw standards than you even know exist.