Eddy, I don't know why BA is recruiting externally, but the very act of this recruitment could be a clue as to what BA is looking for. Maybe BA wants people who are not just like other Pursers who have been in the company for some years but people who will bring something different as they have experience outside BA.
Another clue could be in one of the required skills:
Proactive in supporting and delivering business decisions, cost savings and a strong understanding of business and market knowledge
As Mixed Fleet must make a big contribution to BA's future profitability, it's important to have people who have business know how. I just have a feeling that anyone, which includes current BA main crew, who can demonstrate business awareness will be of interest to BA.