On a misconnect you can be rerouted to another nearby airport <300 miles away. The thing is, you have not yet misconnected. It is predicted that you will, but has not yet happened. BA does like to get all passengers in the air first before rebooking as arrival times can still vary enormously, perhaps you can catch your original flight.

I appreciate this is frustrating, but until your plane is airborne, the exact arrival time is not known and most BA agents, unless perhaps you are calling the GGL line, will refuse any rebooking.