It is possible. Last time I checked a bag there was a group of six people off on a shooting holiday at the desk next to me. Each appeared to have a gun case and were busy having their documentation inspected by the BA agent and one of the HAL (G4S?) agents too.

I expect that whether it can be discharged or not is probably of greater consequence than whether it's considered an antique.
If in doubt I'd say give BA call and confirm exactly what documentation they need you to bring to the airport. Sounds like you have to do that anyway to meet the advanced notification requirement.