The engines may have spooled down low enough to cause the generators to trip off line. The engine auto relight feature would have kicked in I presume and everything back to normal except engines unable to spool up due to whatever!
It was really cold that day, temps were minus 71 degrees C and very un stable, airspeed was all over the place. Still for both engines to wax up within 8 seconds of each other is highly unlikely, also early descent into LHR and maybe holding (did they hold?) would have brought the temps back to about minus 12 ish for landing.
ps Does anyone know whether they tried EEC ALTN or even would it have made any difference IF it had been a FADEC fault? I know that on each start attempt the EEC's (on a 737) use the other channel. Or which thread and page this was discussed?