The frequency used by the VGS at Abingdon is known to Brize, Benson and Farnborough who will and do pass it on, but that assumes the transiting aircaft are talking to any of them after all there is no requirement to in that part of the world.
Agreed the details for Abingdon should be published more, especially as Abingdon is used by military aircraft during the week as well (note the H and parachute symbols on the charts as well as the G). In fact its possible that Abingdon is now busier than it was before the RAF left!
Its also been noted that Abingdon does not appear on some GPS databases, but then again nobody flies with the GPS as their primary means of navigation do they?