The logical explanation as to why a fuel "blockage" occurred (when it did - and no earlier) probably goes like this:
a. Wax globules forming in fuel during cold soak on a lengthy cruise segment would probably adhere to each other, sink to the bottom and gather (courtesy of the fuel-flow's directionality) at a tank low-point (most credibly in the collector box at a pump inlet or filter). Because of mutual adherence making such a clump sizeable (and its icy nature) the wax would
not proceed any further (during cruise or descent) into the Lp (then the HP) fuel system.
b. I'm guessing that there are in-tank bypasses and gravity feed ports to cover such a situation and avoid fuel starvation due to supply-side pump inlet and fuel filter blockages.
c. During the low altitude hold that BA038 was subjected to prior to its approach to 27L, the fuel would have warmed slightly and the mutually attractive coalescence of the icily waxed fuel globules would have broken down and permitted a subdivision of that singular icy clumped wax mass into smaller (and more fluid) waxy globules (but most still with a frozen core). In envisaging this, think about what happens to those gimmicky fluid oil globules as an oil-filled electric lamp heats up and then cools off (i.e. the size of the oil clumps rising and then falling varies with the duration the lamp's been operating).
d. Those smaller detached icy wax globules would have migrated into the HP fuel system as the fuel slowly warmed.
e. Ultimately some of those globules would've been progressively ported into the FADEC's reference ports (i.e. the small diameter lines of those bootstrap labyrinths that, together with various air pressure pick-offs, establish the FADEC's reference datums). The fact that the sticky congealed masses arrived at each FADEC at slightly different times after an autothrottle call for more thrust isn't surprising (but perhaps best explained by the different numbers of oil-fuel heat exchangers in the port and starboard wing-tanks of the 777).
f. In this theory there's no suggestion of any blockage of the main HP or LP fuel supply lines. The blockages would have been in the FADEC's reference port lines (only). As the GE engine's AD at
this link says: "...simultaneous Loss of thrust control events on both engines installed on the Boeing 777 series aircraft due to common mode threats, such as certain atmospheric conditions that may result in ice in the Ps3 or P3B pressure sensing system and causing corrupted signals to the FADEC in both engines."
g. It's not just probable, but obvious therefore from the FAA's concerns, that a "corrupted signal"
will cause a FADEC lockup. Why "wax" and not just ice? Water in suspension in fuel tends to stay that way, however once a fuel "waxes" it takes on an icy constituency, and a component of the mass will be water (in ice form).
h. Questions that remain would seem to include whether FADEC reference port errors actually need a blockage to occur or simply the much increased viscosity of wax causing a "loss of fluidity", a constrained throughput and an illogical scenario for the FADEC's programmed logic. It's my understanding that waxy globules can pass through micron mesh fuel filters. However undeniably also, an icy mass could block both a fine-mesh filter and the alternate path of a small diameter bypass line. It's doubtful whether fuel filters are designed to cope with anything more than the particulate contamination of (say) bad fuel, displaced fuel-tank sealants or a displaced or deteriorating o-ring.
i. The fix most probably will be to warm the fuel picked off to "prime" the FADEC's innards need for data... and to correct for that in the software.
j. Whether a TOGA PRESS would have added thrust because of fuel bypass geometry? That's the multi-million dollar question. Without such a readily available assured solution, the situation remains unacceptably perilous.