San Diego is actually rather a tight airport, as those who have been in there wil be aware. Runway is 9,400 feet but the touchdown point has to be displaced and only 7,500 feet is available when landing westerly (the usual direction) as there are buildings (and an elevated freeway which gives a nice view of things) up against the runway threshold at the landward end. And if taking off easterly you need the performance to get over these as well. Not a fun place for a 777 to have an engine failure at V1.

http://www.airliners.net/open.file/493224/M/

All of this adds up to insufficient length for a 777 with full fuel, passengers and freight for the 5,500 mile trip to London. A 747 can do it but the 777 can't. Standard procedure therefore is to leave the freight off but that ruins the profitability of the flight.