Working currently for a non british oneworld airline it is clearly defined in the SOPs who does the walkaround. It is the duty of the PM of the sector to do the exterior inspection and he signs for that in the tech log, even if it is the FO. However, it is a signature only for the exterior inspection, the captain has to sign the tech log in any case as well.
The FO is as qualified to do the walkaround as is the captain, but i guess that is mainly a training thing, some airlines chose to train their FOs, some don't.