Awair, the initial AAIB report was based on a preliminary readout of the FDR (and crew interviews). The AAIB report states the analysis covered the final phase of the flight. As the FDR covers approximately 700 parameters, if there had been an engine shutdown and/or loss of avionics, one can rightly presume note would be made of such in the initial AAIB report. The only anomaly cited was the non response of the engines to commands for increased thrust.
There is both video and still photography of the plane just before it reached the grass at Heathrow. The RAT is not out.
With respect to this particular incident, the AAIB is fortunate to have data from the FDR and the QAR, covering several thousand parameters. That is an extremely rich data set. It also has a largely intact aircraft, with an undamaged cockpit and quite probably undamaged avionics. If the data points to a potential hardware failure, the suspect equipment can be examined and tested.
In my opinion, the more days that pass without the issuance of an AD or SB related to this accident, the less likely that the cause will be a hardware failure or a software error, at least a failure or error that might be replicated or present in the systems of other 777's.