Interesting line of enquiry, you two, but I would suggest that the resulting figure might be slightly less than the amount on board. It seems likely that the wing bottom surface would have been experiencing positive TATs for at least 10 minutes before the photo was taken.
The aircraft left the Lambourne Hold, according to the AAIB, at FL90. The surface/sea-level temperature was +10C, suggesting something of the order of –8C at FL90. At a typical hold-exit speed of 210kts IAS, I think (in the absence of a Dalton computer) that this would produce a positive TAT. Perhaps someone can tell us? [It is conceivable that the TAT may have become positive prior to the speed-limit point, which would have been 8 minutes earlier. This may have been short-lived, however, and reversed as they slowed down into the hold.]
Time from LAM to touchdown is likely to have been 10 – 15 minutes, during which the skin below the dry part of the fuel tank would have been conducting relative heat towards the ‘wet’ part. This would be eroding the margin of the frost below the ‘wet’ part of the tank, I think. How much would depend partly on the fuel temperature; partly on the TAT.
You should treat whatever figure you arrive with as a MINIMUM estimate. Presence of frost means presence of fuel; conversely, absence of frost does not prove absence of fuel.