If the ATZ is class G, then as long as the aircraft speed is less than 140kts you only need 1500m visibility for an aircraft?
Ah! and there's the rub. (Bold letters where used are emphasised by me)
Guide to Visual Flight Rules (VFR) in the UK published by the CAA
H. Aerodrome Traffic Zones (ATZ)
Pilots wishing to enter an ATZ must comply with the published requirements for that particular aerodrome and in the case of an aerodrome with an Air Traffic Control Unit, with any instructions issued by that unit. ATZs are not included in the Airspace Classification System. An ATZ conforms to the Class of Airspace in which it is situated thus, for example, in Class G Airspace Rule 45 will apply but in Class D Airspace the requirements of Class D will apply in addition (UK AIP section ENR 1.4 refers).
A. Weather minima VFR flight within Controlled Airspace (Classes C to E Airspace)
• As per below FL 100….or… • Fixed wing aircraft operating at 140kt or less: 5 km flight visibility; Clear of cloud and in sight of the surface
Special VFR Flight
A Special VFR flight is a flight made in a Control Zone under circumstances which would normally require the flight to be made under the Instrument Flight Rules (IFR) but is made under special conditions and with the permission of ATC instead of under the full IFR
(b) Weather limitations for Special VFR flights arriving at or departing from certain aerodromes may be detailed in the UK AIP. Without prejudice to such limitations, ATC will not issue a Special VFR clearance to any fixed wing aircraft intending to depart from an aerodrome in a Control Zone when the official meteorological report indicates that the visibility is 1800 m or less and/or the cloud ceiling is less than 600ft
By doing a very short survey using the UK AIP: it is common practice for operators to align their ATZ with the class of airspace within which it sits. So, has the privileges of the SVFRs been extended from that of transit; and therefore a request for circuits subject to SFVRs becomes permissible.