Upvote:1
National Rail Conditions of Carriage (PDF)
C. USE OF TICKETS
16. Starting, breaking or ending a journey at intermediate stations You may start, or break and resume, a journey (in either direction in the case of a return ticket) at any intermediate station, as long as the ticket you hold is valid for the trains you want to use. You may also end your journey (in either direction in the case of a return ticket) before the destination shown on the ticket
Upvote:2
It depends on which train ticket you buy.
The Off-Peak Saver Return on Birmingham Stations to Banbury permits a break-of-journey on the outbound portion, so you could join the train at Birmingham Airport.
I suspect the cheaper off-peak saver return ticket on Birmingham Airport to Banbury would not allow you to go so far as Birmingham Stations, but I haven't checked in the routeing guide. If it did allow this, it would be a cheaper alternative.