Peter Smyth wrote

"elyob" wrote
[...]
I'm guessing it should be okay, but am wondering whether the
barriers are
clever enough.

There are no valid routes from London to Brighton that pass through
Wimbledon so you could not exit there. However you could use your
season
ticket to exit at stations on a valid route, eg East Croydon.

But you might have to go to the manual gate, since, as the OP suggests,
the barriers are not always smart enough.

Years ago, the Clapham Junction barriers would not allow exit on a
season ticket on my way back from London to Surrey but this has now
been fixed. More recently on returning to Woking from Bournemouth the
SDR would not allow exit at Farnbrourgh (Main)'s barriers.

--
Mike D