Far less likely to convince a guard that Droitwich Spa - Chippenham is valid via Birmingham
I agree there is always a chance that one day you'll get a TM not prepared to accept the ticket. I've had a couple of TIRs issued, but correspondence with CrossCountry confirmed my original tickets were valid. A friend successfully appealed a UFN also. You can of course protect yourself further by getting an itinerary and seat reservations, as booking engines will give both for a Droitwich Spa - Chippenham Off Peak Return.
Incidentally, despite Easement 700189, you can also get seat reservations for Worcester and Malvern to Chippenham via Birmingham New Street using booking engines.
Someone not confident enough to use the 'system' to their financial benefit is of course better off (piece of mind, if not financially) with the Craven Arms - Teignmouth option.