Hi Bookworm,
VFR/SVFR criteria are available in the AIP and also in MATS Part 1 1-5 to 1-7. Yes, technically speaking if you fly at greater than 140kts you are required to be 1500m horizontally and 1000ft vertically clear of cloud in Class D to remain VMC, but the visibility criteria remains 5km. When the visibility drops to 5km or less at City Airport, we ask pilots whether they require SVFR or VFR clearances.
Since the Specified Area was established many years ago (at least 30), the congested area of Central London has expanded considerably with no change to the dimensions of the Specifed Area. I agree according to the ANO that it applies nominally to single engined helicopters, but the CAA Legal Branch seem to see things slightly differently, taking a view that there is no way a fixed wing single flying SVFR at 1500ft over Central London could alight clear in the event of failure of a power unit. Folklore has it that one day a "legal eagle" happened to spot such an aircraft from the window of Aviation House, took tracing action and prosecuted the pilot! Although I am unable to refuse a SVFR clearance over the specified area to a fixed wing single, I would STRONGLY discourage a pilot from requesting such a clearance as it puts myself and my colleagues in a very difficult position. It is a grey area indeed and one probably best avoided.
The PAPIs at City Airport are set at 5.5 degrees. What I meant to infer is that pilots will not get the glidepath information they are used to if they rely on the City PAPIs.