I wonder what the consequences might have been if BEA had remained at Northolt, the runway realigned to E/W and a tunnel build to Heathrow. Still hypothetical now.
..........or if a third (and fourth) rwy had been built as soon as it became apparent that it was required, i.e. at about the time when the government declared that Heathrow was "full" (circa 1977).
Equally hypothetical?