Lovely 1000 metre strip at Thame (just by Haddenham), on the chart and in the guides, but usually only available at weekends, when it is used by a gliding club (NB winch launches). The arrangement used to be that the owners (who are never there) welcome powered visitors, with no PPR. Non radio, circuits to the north, avoid the villages, give way to the gliders and notice where the winch tractor is, landing on the other side of the wide runway to avoid the cable when it is on the ground. In reality the gliding people who use the field are not that pleased to see powered aircraft (I'm always a bit saddened that we could be one sport, but are many) but, as I say, the owners say "welcome". NB absolutely zero facilities, not even a portaloo. No windsock.
Otherwise Chalgrove, Benson if you have connections and loadsa insurance, or Wycombe, or Oxford. There is a farmstrip near Stokenchurch, but I assume it is very private.