Depends on the station and route - some have gates, few now have manned barriers, some have nothing. And some trains have conductors, where others have spot checks by revenue protection teams.

Basically, if you need to show your ticket to a person, whether at a station or on board a train, you need to have your railcard with you, or you'll be treated as if you don't have a ticket. No leeway.

Have fun - hope the railway gods are smiling on you...