And to save any confussion, yes each pilot generally takes it in turns, i.e Capt flies as PF for the first sector and F/O flies as PNF, on the return leg they swap.
F/O Acts as Captain when it is his leg making all the decsions BUT the butt always stops with the Captain. i.e if the F/O screws it up it is the Captain's fault.
bmi do not have a very good crewing department so crews spend alot of time passengering around the network. I have absolutely no doubt that the F/O you saw was not operating but had just gone in to see the operating crew!!!!!
Happy flying