BAs procedure is to get the Take Off figures, via ACARs, as they taxy. I have waited for short periods of time behind them whilst they receive and enter the numbers into box. I'm not saying this is a factor in this particular incident, but the pilots cannot be concentrating fully whilst either waiting or loading the numbers and taxying?
It's a big black hole down that end of the taxiway, and 3 pairs of eyes is always a must.