Roland and Beags have it - the key issue obviating London expansion at Rissi and many other former stations is access.
Conversely, not only does Upper Heyford have remarkably good road and rail infrastructure already in place, but the travel time on existing rail from central London is less than that to the nether LHR terminals on the Tube! What's more, the terrain would allow for the construction of at least one more 260 with the requisite separation, and all this in an area of minimal population density.
To my simple mind, redevelopment of the wickedly-wasted Upper Heyford facilities (perhaps with further enhancement of the Chiltern line, instead of the wanton folly of HS2) is just a no-brainer .... but, as we all know, Whitehall is even lower that in the brainpower stakes.