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.
The future of the A4/Colnbrook Bypass seems to be in doubt too. The original NW plan had the A4 arcing south to the new roundabout east of Poyle, then heading east in a tunnel under the taxiways to join the current Bath Road at the West Ramp entrance (though how it would cross the M25 is anybody's guess).
The revised NW plan appears to sever the Colnbrook bypass, leaving no route for existing A4 E-W non-motorway traffic other than through the middle of Poyle and Colnbrook. It's hard to believe that's what the planners intend.