Pioneers get arrows in their back and schedules do slip. G.fast technology is rapidly improving, with Amendment 2 and Amendment 3 chips delayed until the second half of 2017. BT had originally promised to start their 10M home rollout late this year or early next. In particular, BT wants 48 or 96 ports while today's tech only supports 16-24 ports. They also want the higher power and longer reach of the Amendment 2/3 chips. That's particularly important. Their last goal (350 megabits down at 350 meters) is beyond the current state of the art. It may be impossible, but that's unclear.
The original plan, promised by CEO Gavin Patterson, would go to local distribution points for speeds generally 500+ megabits. The 500 meg promise was part of Gavin's attempt to persuade OFCOM not to break up BT because they would deliver great Internet to "nearly all of Britain by 2025." Then the bean counters jumped in and said they wouldn't pay for the high speed. Using the existing cabinets brings the cost so low it fits into the existing capex budget but also reduces the speed.
Some very good BT engineers redesigned the project for 300 meters rather than 100 meters.