A landing threshold displaced by 700m, all other things being equal, makes about 120' difference to the height of an aircraft at any point on a 3° ILS (though said document also makes a reference to "steeper landing approaches").
I believe they were intending to use a 3.2 degree approach in addition to the displaced threshold to reduce noise disturbance. The summary document includes an estimated noise profile chart. I certainly remember reading that 747's and the like would not be allowed at Heathrow by the time the third runway, if approved, was built.