Special VFR is given ONLY during the day and only to a minimum 600 foot cloud base and 1500 metres vis.
There are differences. One being that contrary to ICAO, the UK does not require all flights to cruise at an absolute minimum of 500ft. That is where the 600ft requirement comes from under your rules and ICAO i.e. if the cloud is less than 600ft then how can you ever cruise at 500ft and not be in the cloud.
The daytime limit is your own local rule.
Regards,
DFC