Maybe less to this story than the headline suggests..The Gibraltar Chronicle has reported that the Spanish P3 was transiting Gib airspace, under the control of Seville ATC. which it is entitled to do. At some point it did make an alteration of course, presumably ok`d by Seville because the P3 then contacted Gibraltar ATC (unusual for the Spanish Military) and explained their actions.
At that point the BA flight resumed its flight path to Gib and the P3 continued on out of their area