There is much confusion.
Category Z flights are non-standard flights such as photographic details.
A to B flights which have filed a flight plan in the normal way are simply normal flights. They do not have a lower priority. A simple test of this being if they were IFR, they would get a CTOT or a no delay requirement to depart within 15 minutes just the same as every other flight.
Special VFR flights are at the discretion of ATC at all times and in all cases or categories. That is because SVFR is in place at ATC discretion to permit aircraft who can not comply with the IFR requirements to enter the zone when it is IMC or when it is night or when it is class A - Heathrow.
IFR flights can operate below 3000ft, clear of cloud and in sight of the surface and not be required to comply with the 1000ft above obstacles within 5nm rule. Note - no visibility requirement!
Reflex, I think you mean 2000kg and you are correct - every heli above that weight pays.
Thus while I agree that if SVFR is closed then no calls for cutting the corner or SVFR transits to White Waltham via Ascott should be entertained, operators who are flying commercial air transport to and from Battersea should be accomodated either IFR of SVFR on a different frequency.
As I said the airspace is for the protection of traffic. Not for the exclusion of traffic.
Regards,
DFC