Oh, it reads in inches because the E-3 was bought at short notice pretty much 'off the shelf' following the AEW Nimrod fiasco. Unlike so many other US (as opposed to U/S that is) bits of kit, 'we' didn't have time to meddle with it, at great expense, to convert to UK standard.
Sadly, many, if not most, E-3 crew don't perform their own conversions.
Not so much in the UK, but certainly in europe, I've had many an air traffic agency convert mb to inches incorrectly. I have seen E-3 crews accept an incorrect a/t figure rather than trust the conversion out of the FIH. W@nkers.