11 miles out @ 530 feet AGL

[PJ2]

BOAC;

Apart from 'lost' or some misguided 'dive and drive' approach, does anyone have any idea why they descended before the FAF?

Yes. I provided three separate posts, (here, here and here) on ideas why they descended before the FAF and provided links to two EGPWS documents. I said that I had recently seen this occur once again and gave the reason why it occurred, (glidepath failed to the centered-position - crew followed it).

TAWS (EGPWS) and, I assume T2CAS, have Premature Descent Alerting (PDA) if the aircraft is in the landing configuration but descends below a nominal, internally-generated 3deg glidepath. Inside 12nm, a "Too Low Terrain" warning would not occur until 400ft RA.