As you are fully aware the bus no longer runs why would you buy the ticket except to try and be a smartarse!
We know this is the case here, but for a general punter who doesn't know it no longer exists, how would they stand when they get rejected by the bus driver.
there is 4 buses an hour from the railway station to the bus destination he is querying, 2 from western greyhound, 2 from First. A third operator has a 4 buses per day service. This third operator took over the route that the ticket was valid on and cut frequency to make the route viable. The ticket says valid on a bus route that ceased around November 2013, but FGW are still selling the ticket through their ticket sales facility.
Depending on the original posters origin there may be cheaper options anyway if he is looking to save money,