The 1800m limit is indeed
mandatory! From the ANO:
The rating does not entitle the holder of the licence to fly:
(a) on a special VFR flight in a control zone in a flight visibility of less than three km; or
(b) when the aeroplane is taking off or landing at any place if the flight visibility below cloud is less than 1800 metres.
It has already been suggested that the 1800m limit is harmonised with the 1500m VFR limit - and it's also clear that the 3 km limit should be reviewed, given that there's a SERA derogation for 1500m SVFR under specified circumstances - see ORS4 No.1078.