The 1800m limitation (vis required for take-off and landing) is published in Schedule 8 of the ANO, where the rating is defined.
The ceiling limits are published in the AIP, under the Aerodromes-General section AD 1.1.2. (Section 3.3.2)
Whereas most people are clear about the 1800m rule, the ceiling rules seem to cause confusion, with many people believing they are advisory only.
The advisory aspect is the advice to add 200 to the IR minima for a precision approach. However there is an "absolute minimum" for the DH of 500 feet. Similarly the "absolute minimum" for a non-precision approach is 600 feet.
My understanding is that these "Absolute Minima" are, as the name implies, legally binding.
Hope this helps.
2D