About UK Milliradians
The UK milliradian is one thousandth of a radian, or 0.0573°, so 6,283 of them fit in a circle. At 1 meter it spans 1mm.
An object that fills 1 milliradian of your view is 1 meter wide at 1km, or 1mm wide at 1 meter. Divide the width by the angle to get the range – a 1.8m person covering 3 milliradians is 600m away.
Degrees don't give you that shortcut: at 1km one degree is about 17.45m wide, which is nobody's idea of a round number.
About US WW2 Milliradians
The US wartime mil was an artillery unit, a rounded milliradian, and a right angle came to exactly 1,000 of them (4,000 to a circle). A true milliradian is smaller: 6,283 fit in a circle.
In World War II American artillery crews set elevation and direction in mils, reading the angle between the target and the gun from an aiming circle.
There are 4,000 US WW2 milliradians in a full circle.