It is just worth asking the Guard. Perhaps you might have to explain to the Barrier staff but is definitely worth a go. After all the Guard is human and will definitely appreciate you asking them first.
I'm afraid that's not always a given! I was due home on
SWT▸ from Waterloo to Honiton yesterday, but the SWT line was shut due to 2x fallen trees. We were advised to use the Paddington service instead and that our tickets would still be valid.
Of course, the Paddington line is currently diverted via Yeovil Junction/Honiton anyway. Our train stopped at Honiton for over 10 minutes (single line) but the guard flatly refused to let any Waterloo refugees alight there! We were forced to travel on to Exeter St. Davids where
FGW▸ promptly laid on a taxi back to Honiton, utter madness!!!