Bookworm, agree with you entirely on all counts. The 1500 ft part of Rule 5 should be the overriding consideration. The complication is that the Specified Area overlaps both the London Control Zone where the the 1500ft Rule doesn't apply (to aircraft on SVFR clearances), and the London City Control Zone where the Rule does apply to aircraft on VFR clearances. Having said that, fixed wing and helicopter twins are instructed to fly AT 1500ft SVFR in the sector 020 degrees to 140 degrees true from Heathrow Airport (somewhat wider than the Specifed Area). This I suppose gives them a "fighting chance" of alighting clear in the event of power failure. If twin helicopters are unable to maintain 1500ft they are unable to route direct and must use the heliroutes. Single helicopters are catered for by the network of helicopter routes which end up following the River Thames when they reach the Specifed Area.