Yes - it is on age 19 of GASIL 2002/03
It didn't say you couldn't fly down the Lea Valley, just that you must be able to alight clear. The full text:
Single Engined aircraft have been seen flying over the built up area of greater London, in the area of the London City Control Zone. While the map shows area which appear to have no major buildings (for example along the river Lee, to the west of the aerodrome), this is still regarded by the courts as being a 'congested area of a town, city or settlement' for the purposes of Rule 5 of the Rules of the Air Regulations 1996. Pilots have been prosecuted and penalised for flying over that area at a height below which would allow the aircraft to alight clear of the area and without danger to persons or property on the surface, in the event of a failure of a power unit, as required by Rule 5(1)(a)(i).
An air traffic controller may give a pilot clearance to fly on a special VFR flight through the control zone. Such a clearance entitles a pilot to disregard Rule 5(1)(a)(ii) (so that he may be below 1500 feet where so instructed by ATC) but not Rule 5(1)(a)(i) (the alight clear requirement). It remians the pilot's responsibility to be able to comply at all times with Rule 5(1)(a)(i) regardless of whether or not he has been issued with a special VFR clearance.
GASIL 03 of 2002 - September 02
Doesn't guide you on whether or not 2400' is high enough to escape!