NSEU
The leakage would have occured after the spar valves, perhaps in the strut area, prior to the fuel pumps and FMU: The FMU has a high pressure shut off valve, so if this was closed, the leakage would have to be between the spar valve and the HP shutoff. Even if the FMU HPSOV was open, the fuel would have a bit of a stuggle to get throught the engine pumps, especially with the tank pumps off and the engines not turning.
Look at the state of the engines. They were badly damaged. The AAIB report specifically mentions that although the spar valves were turned off they did not close because the wiring was damaged by the undercarriage failure. So the fuel in the engine feed lines was leaking out through the damaged engines onto the ground.