If BA has provided for a delayed check-in, you may rely on that. If BA does not provide for such a delay, you are nonetheless required to meet deadlines as originally scheduled.

This is because many aircraft make up time enroute, the turn time on the ground can be reduced and aircraft substitutions are common.

Here, BA appears to have made the decision that it will not substitute aircraft and that the new departure time is fairly baked into the system.

One other consideration. If the delay is occasioned by creeping problems which may lead to lengthy delays/cancellation, being present at the airport where you may be able to divine better information from staff at the airport, getting yourself rerouted is easier sooner rather than later. If only 20 seats on some other routing, you may be sitting in a car service while others are having new tickets validated.