About Degrees
A degree (°) is 1/360 of a full turn, and an angle is measured by how far one line has rotated from another. The 360 is generally credited to the Babylonians, whose base-60 counting made it a convenient choice.
Each degree is split into 60 minutes (′) and each minute into 60 seconds (″), so 1° is 3,600″.
Multiply radians by 180/π to get degrees – 1 radian is about 57.3° – and multiply degrees by π/180 to go back, so 180° is π radians.
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.