FWIW, ExpertFlyer shows it as a 787-10 and with half the First seats already chosen.

It also has this as availability for BA49 on 9/9/23:

F4 A0 J9 C9 D9 R9 I0 W9 E5 T0 Y9 B9 H9 K9 M9 L9 V9 S9 N9 Q9 O9 G0

The fact they're still happily selling First seats, and that four First passengers have chosen their seats already, would suggest it's not been swapped with a Finnair plane (as they don't have First!)