Just out of interest to anybody but
FGW▸ do offer Carnet Tickets in the Oxford area, again from unmanned stations and a couple on the North Cotswolds as below :
Kings Sutton
Heyford
Tackley
Radley
Culham
Appleford
Hanbrough
Combe (Oxon)
Finstock
Charlbury
Ascott under Wychwood
Shipton
Kingham
And also Chiltern Railways gained the Bicster branch so they are also avaliable from:
Islip & Bicester Town.
Yet to see them advertised officially by FGW but the regulars use the magic word of mouth so that seems to work.
However there are terms and conditions to these tickets which you can get from ticket offices and they are also printed with the tickets if purchased from a STAR machine unlike those produced from AVANTIX machines on train.
Also
FCC▸ have a carnet system in place link here :
http://www.nationalrail.co.uk/times_fares/prb225ca2e6e40ef93d44bccd007b205.aspx