I believe that there is an official doubleback between Barnt Green and University stations, therefore if the OP was to change at Cheltenham Spa onto the Cardiff - Nottingham service, it would be acceptable to alight at University and double back.
There are a few easements in place, but I don't know of any suitable ones. Of the easements which name Bromsgrove or University:
- 300730, 700128, 700248, 700331, 700444, 700445 and 700474 are only valid on tickets from or to places further northeast;
- 700351 and 700645 only affect "any permitted" tickets (which are much dearer); and
- 700330 only affects journeys via Severn Tunnel Junction, which would probably be slower than via Worcester, especially with the Severn Tunnel being closed that day.
That said, you could use 700330 and miss the tunnel bit out. Bath-Cheltenham is £14.30 off-peak, and Patchway-Bromsgrove £13, to make £27.30. In full, 700330 says "Customers travelling via Seven Tunnel Junction (sic) to Bromsgrove may double back via University. This easement applies in both directions."
Furthermore, as the West Midlands PTE area has concentric zones (similar to London), Zone 1 contains New Street, Moor Street, Snow Hill, Jewellery Quarter, and Five Ways. University is in Zone 2.
That's something I hadn't thought about. The new rules (which come in on Saturday) will mean you can combine zone-based tickets with other tickets without worrying about where trains stop. That said, you'd need an out of county ticket to Bromsgrove, and I don't know how much that would cost.
