Makes sense, have a look at the old plan to the new one keeping the current M25/M4 Junctions.
The traffic engineer in me (but evidently not at BAA) tells me that the revised road plan will never be accepted. The high level links from the eastbound M4 to the southbound M25 (think doing Reading to T5), which is the top level in the M4/M25 junction, will never be able to be got down beneath the runway level in the space shown. It is already at maximum descending gradient for a motorway.