I think it was something to do with the plans showing the then standard RAF wartime airfield triangular runway layout, to maintain the fiction that LHR was intended to be a new RAF transport aerodrome, hence all that wasted money (and £400,000 in 1944 would equate to vastly more in today's money of course). A very interesting book, "Heathrow, 2000 years of history," for a propliner enthusiast with some environmental leanings-myself.