I believe that the figure of 6,900kgs was what the crew had entered into the FMS so as to generate an "insufficient fuel" message. This figure does not include the contingency fuel or any extra fuel (a nice round figure of 79,000 implies that they probably took some extra). If everything went to plan then this contingency fuel, and most of the extra fuel would still be on board at LHR which explains the recorded figure of 10,500kgs at 1000ft.
Perhaps a 777 pilot could confirm that this is correct.
Airclues