About Centiradians
The centiradian, or centrad (crad), is one hundredth of a radian – the radian with the prefix centi- applied.
A full circle is 2π radians, which is about 628.3 centiradians.
In practice small angles are given in milliradians or arcminutes rather than centiradians. 1 centiradian is 10 milliradians, or about 34.4 arcminutes.
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.