When I looked at the promotional photos we got from S2, I realised there is a deleted AM scene from episode 202. I hope when can get the footage of that scene someday (20th anniverary special edition?) Anyway, this is what I think happened in that scene.
Photographer: Bradley, Colin, how are you today?
Colin (turned around and spoke with his real accent): Very good.
Director: You two find a corner later. NOW - ACTION!
Bradley & Colin switched to Arthur & Colin
Photographer (busy with his camera); Bradley / Colin spoke their lines.
Photographer zoomed out. Merlin/Colin still pretended to help with Arthur/Bradley armour.
Director: Cut!
Arthur & Merlin switched back to Bradley & Colin.
Photographer (spoke to himself): This is pure gold!