Hi Bren,
Yes, correct as usual, as mentioned in the "lining up from the South" thread which has mysteriously disappeared from here! To cross at the threshold of 27L aircraft would have to hold at the CAT1 holding points in Blocks 94 and 95 which are a long way from the runway. As the aircraft began its crossing it would infringe the 27L glidepath causing severe deviations to the glidepath signal, so you would need about a 15 mile gap to cross safely in IFR conditions! MLS will solve this particular problem.
Block 85 is another option of course, but the nearer we can cross to the threshold the easier it is to achieve the manoeuvre, and using block 85 will conflict with inbound Tango and Sierra traffic. Terminal 4 noise and health and safety restrictions do not contribute to an expeditious operation either.