Schedule 8 won't help you (it doesn't mention DH/MDH).
Art 40 makes it mandatory to use the 'specified decision height' (or equivalent) as notified in the AIP.
AIP AD 1.1.2 para 3.3.2.1 says that IMC rated pilots "are recommended to add 200 ft to the minimum applicable DH/MDH, but with an absolute minimum of 500 ft for a precision approach and 600 ft for a non-precision approach".
The only interpretation of that that might support the 500/600 ft being mandatory is if absolute is interpreted as mandatory in contrast to recommended. There is little justification for interpreting absolute in that way. It means not relative, in contrast to the increments in the previous part of the sentence.