The London Heathrow control zone is class A airspace and there are published helicopter routes to various places within that airspace. They are all SVFR routes. If you call for zone entry via a route the clearance you receive is normally something along the lines of "Cleared H3, Standard operating altitudes, Special VFR, report at XXX."
i.e. Pilot asks for a clearance to enter the zone, giving details of the required route. ATC read back clearance incuding the words "Special VFR", so I guess the protocol is that they offer (only) SVFR. It would be hightly unusual to ask for an IFR clearance unless previously written flight planned and impossible to get a VFR clearance in Class A. So SVFR it usually is but ATC instigate the actual words.