Looks like route 'SOUTHERN&AGA' fares have been withdrawn.
I have been aware of people using these fares from Cambridge from their introduction in NFM08 (see
FareHistory) and I do wonder, why would GTR want a fare that abstracts money from them by putting people on a rival operator between Cambridge & London?
That said, GTR should have replaced this with a ticket valid on services operated by GTR only, and they could have keep 100% of the revenue.
Technically no fares should be referring to "Southern" any more as it's not TSA compliant; if there was a will to retain it, it should be "GTR+AGA", which would effectively be "Not SWT" 