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!
In case I'm misunderstood, let me just clarify. I think the area which a fixed wing single must be able to glide clear of is substantially greater than the Specified Area, i.e. that the rules are much stricter for fixed wing than helicopters.
Rule 5 says in general that:
helicopters shall not fly below such height as would enable it to alight without danger to persons or property on the surface, in the event of failure of a power unit;
but:
an aircraft other than a helicopter shall not fly over any congested area of a city, town or settlement below_such height as would enable the aircraft to alight clear of the area and without danger to persons or property on the surface, in the event of failure of a power unit
So generally, while a helicopter flying over a congested area only has to be able to find somewhere in the congested area to put down safely, a fixed wing aircraft must be able to clear the congested area entirely.
(The exception for the helicopter is the Specified Area of London, which it must be able to alight clear of in its entirety.)
Since a congested area "in relation to a city, town or settlement, means any area which is substantially used for residential, industrial, commercial or recreational purposes" it seems unlikely that there is a much wider area of the city (including, but not limited to, the Specified Area) over which a fixed wing at 1500 ft is not going to be able to glide to a site completely clear of the "congested area".