I don't think the final figures are to blame here. Remember, there were three pilots on the flight deck on this flight. Quite often the 'heavy' will deal with the figures.
Also, BA has recently changed the SOP's on the 747 substantially. One noticeable upside of this was the removal of nearly all distractions during the taxi phase. (I.e. the flight controls are now checked before taxi etc).
This was just a simple mistake in a very dark area of the airfield. Unfortunately, the consequences were severe. Looking at the photos on here, it is a very badly designed portion of the taxiway.