Why not pick the one with the most runways, Bedford Thurleigh! Hasn't it been suggested before?
Yep, several times. Problem with Thurleigh is, again, access. It might be a short spur from one of the main N-S railway lines but as anyone who has lived around Bedford will tell you, traffic is a nightmare. Last real attempt, IIRC, was to turn it into a freight airport with good rail links, but for road links you are pretty much stuck with single carriage way and invariably through, rather than round, small villages.
Can't help thinking that Alconbury and Upper Heyford would make much better new "London" airports, particularly UH with its proximity to the M40 and another N-S mainline.