The last time I checked, neither Turweston nor Northampton have had commercial passenger service available to the general public for a very long time - I very much doubt they are capable of handling commercial passenger aircraft on a regular basis.
Minoan may not be around for much longer, but Oxford does have normal commercial scheduled passenger flights. If Cranfield and Oxford are, as you say, equidistant from Silverstone, then it seems fair for Oxford to claim to be the nearest airport to Silverstone.