I thought that GASIL was trying to point out that the Lea Valley was not to be considered as an area to be used if you need to alight clear. I thought that the second paragraph was just a reminder of the rules.
To answert he question, no - we don't get many SVFR transits in the City Zone. As stated above, it would need to be below VFR mimima or at night. Not too many pilots can fly at night and not many can fly in poor weather!
The reason that it is tricky to give a SVFR clearance is that we have to provide standard seperation between SVFR-SVFR and SVFR-IFR (3nm or 1000 feet). This is the porblem with the London Class A zone - one aircraft routeing on the BUR/ASCOT route effectively sterilises a huge amount of the zone - esp for opeeosite direction traffic. Given the small dimensions of the City zone, during inbound/outbounds it would be near impossible to achieve standar seperation. It is much easier to issue VFR which only needs traffic info on IFR. (the IFR can ask for traffic avoidance if he wants). You may have heard the "Hold at the Southern Most tip of the Isle of Dogs - report when visual with the 146 departing 28 at City" - followed when you are visual with "with that in sight continue northbound not above 2400 feet".
Hope that helps