I'm with dan atki and Tom C on this issue and I don't agree with hairyhandedfool and jv3531.
I'm only going on what Tom C said as it's his line of expertise

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If the journey seems reasonable via BHM (and if it is showing up on journey planners with the same fare you have no reason to suspect it's not reasonable).
Without looking it up formally and giving my take on it, BHM isn't a million miles away (which East Grinstead was in Tom C's post where the off-route/beyond validity debate started) and if that is the quickest route (not necessarily the shortest) and you can explain what you're doing there then many inspectors will agree that it is reasonable.
Please forgive me for using 'reasonable' against 'valid', however, as the two are completely different - not that I'm commenting on the validity though - having not checked properly (either RG, maps, nor timetables) I can't comment on validity.
What I would say though is if it is deemed invalid by the inspector, ask them what they would deem to be a valid route for the journey, as you thought you were travelling on a legitimate route.
Done a quick search on NXEC, however, and it shows the same journey time and same fare for changing at both Wolverhampton and Birmingham. If it is deemed you are off route, the excess given should be £0.00 (can Avantix Mobile specify via stations?).