Another potential benefit for SOU is that these flights should be pretty visible to BA's London customer base, perhaps more so than any flights operated by other carriers from SOU.
Head for points and other websites will be read by a lot of business travellers in the London area, so now they'll know that there are holiday flights from the airport down the road that they can spend and acquire Avios points on. This may entice a few more SW London passengers to drive down the M3 or jump on the train than may otherwise have been the case.