The only thing that comes to mind is the Little Chef just south of the junction with the M40, but check its still there, as they've had a big sort out. Can't think of a Greasy Spoon in Newbury, and the villages are far too posh - you'll only get almond croissants.
Spot on there, nothing in newbury (I live there). There is a truck stop off the a43 jus before silverstone but that may be too early for you.