My understanding was that P4 would be double length and extend into the trainshed, rather than P5 being shared but I guess there were several versions of the plan.I've never heard that, - how would it work, - ES requires a 760mm (ISTR) height whereas UK domestic would be something like 915mm? Unless there was a minimum width MML platform squeezed in on the western side and what is now P5 ES was slightly narrower.
Maybe the Home office objected because an athletic migrant might take a run at it and jump "illegally" into the UK landside.