During a monitored approach PF will not land the aircraft. PNF will take over at DA (or before if visual). At DA PF will say "decide" and PNF will either say "Land, I have control", or "Go around". If go around is decided then PF will fly the go around (there will be no change over of roles).
I don't fly for BA, but that's how the monitored approach works in my company.