For the 777 which takes 400 feet to select gear down to locked on a 3 degree approach, and then 400 feet to go from flap 20 to flap 30, you need 800 feet minimum. You should not select flap 30 with gear still travelling as you get a config warning. So 2000 feet AAL is the BA (and other) recommended latest selection of gear and 1500 feet AAL latest flap 30 selection. Vref30 will be sigificantly below 160 kts, requiring power down and time to stabilise and then spool up for the 1000ft AAL gate.
Well almost - if it helps to clarify matters, intermediate approach power settings are in the order of 55% N1 with Flap 5 selected, 180kt. With a 10kt headwind component, the 777 will happily fly a 3deg slope with Flap 15, 160kt to 4 DME. Power settings at this point will still be quite low (<40% N1 with speed stabilised). At about 2000ft RA the gear will be selected down (As jet999 correctly states, BA SOP is to have gear selected down by 2000ft RA (radio altitude=height above the ground), and configured in the stabilised approach
BY 1000ft RA. The onboard flight data monitoring system (called SESMA) will trigger any significant divergence from these criteria.) Flap 20 will be the next called flap setting, and there will be a marked thrust increase to account for the large extra drag of gear and flaps to about 55% N1. At approximately 1500ft RA, selection of final flap 25 (BA standard landing flap setting, not 30) will result in a small thrust
increase from gear down, Flap 20 (due to increased drag - the a/t absorbs the speed decrease of approx. 15kt using the extra drag to reduce thrust lever movement), so that final configuration is achieved and stabilised by 1000ft RA. The SOP trigger for a go-around would be the 500ft RA call - if not in the correct configuration, profile and speed a go-around must be flown.
My point is that by 600ft RA the aircraft should have been correctly configured and with approach power (~55% N1) set. A loss of thrust lever command authority at 600ft RA would only be significant if the thrust setting was towards idle to begin with, and didn't return to the normal 50-55% required with the aircraft in landing configuration.
Either the reporting is incorrect and the fault was identified earlier when the engines spooled up on final flap selection, or the approach was a bit too slick, with the thrust only commanded to come up from <30% N1 to approach power just before the 500ft gate.
The conclusion from the latter case would be that although they mitigated their unfortunate situation extremely well, they could have avoided it altogether by being correctly configured a bit earlier. Engines stuck at 55% N1 are a bit more useful in flight than stuck at 30%!