So in other words.
To get fibre to the home wont cost £28.8bn it'll cost £5.76bn.
If 80% of the £28.8bn is in street digging then that cost is wrong because you don't need to dig the street. Fibre via sewers, overhead fibre, the method a company advertised years ago that just cuts a narrow fibre width cut in the road whilst laying the cable behind it and filling it in afterwards so that you can lay miles of the stuff in a day with 0 disruption all do away with well over £20bn of the £28.8bn figure.
Quite why therefore the £28.8bn figure is being banded about I don't know, the only reason I can guess is that the person behind this report has some vested interest in not seeing next generation broadband rolled out because again, the real figure of deployment isn't even a quarter of this amount he has stated. Presumably also fibre to cabinet can also be decreased by a similar ratio dropping the cost of that to around £1bn.