It all depends on the airspeed at the time things started to go wrong!! Vat would probaly be around 140kts. This is an assumption obviously. At 2DME the 777 would already be slowing down from 170/180 knts in order to achieve Vat. Probably full flaps would already have been set. My estimation is that the engines were working at say 10-15% above idle and didn't respond when more power was requested.
Look here for the initial report
http://www.aaib.dft.gov.uk/latest_news/index.cfmAlso this website will eventually publish the full report.
They were lucky they evidently hit the ground very hard which suggests the a/c stalled in say from 20feet. If they hadn't been able to get over the hedge who knows what would have happened!!
Vololiberista