In the days of paper tickets, endorsable meant that it could be "stickered" to be usable on another carrier, for example if you were rebooked during IROPs. Not endorsable usually meant award tickets that would only be good for flights operated by the same carrier. Today with e-tickets, even award tickets tend to be rebooked at least onto partner flights (or any flight operated by an airline with which the carrier interlines if the problem is the fault of the original carrier, such as a mechanical delay).

To take another earlier flight at your option would require a flexible ticket, so check your fare rules. Some carriers will rebook you on flights the same day, on either a confirmed or standby basis, but BA almost certainly won't unless your flight is delayed or cancelled.