About Sign
A sign is 30 degrees, one twelfth of a circle, or π/6 (0.5236) radians. It comes from the 12 signs of the zodiac, each covering 30° of the ecliptic, and the word is from the Latin signum, a mark.
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.