That debate has taken place over the years, and if I recall correctly the problem is St Mary's Church in Stanwell Moor and the village, which would be impacted, if not destroyed.
A better solution would be to build it due south of the King George VI reservoir. To save anyone looking it up, that would mean building it where Staines is now, as apart from the Arapaho Spur, Staines has nothing to commend it!