MJ and CGB, I know exactly what you are both saying BUT the point you perhaps both miss is the "clear of cloud" part. Pilots must comply with both requirements. This is a separate issue from the 800 metres visibility limit for the IFR "easement/alternative" below 3,000 feet, or from the 1500 metres required for VFR.
The relevant limit of visibility can prevail without the presence of any cloud. In this case there would be no advantage in climbing to the base of airspace, it's just poor visibility, possibly so all the way to the ground.
However, if operating in these low vis conditions, in some areas it may still be necessary to climb close to the base of airspace above in order to comply with the low flying rules (i.e. over congested areas on high ground).
There was originally no limit of visibility specified for RW, the requirement for an in flight visibility of 1500 metres for VFR came in only a few years ago.