Barcelona fares are noticeably cheaper than the other Spanish/Portuguese 'sun' routes from SOU although I expect this is as it will attract more a mix of city-break and holiday pax, and is yet to be well established. Also worth considering for most of the dates I have looked at there is nigh on no difference between the price of Flybe from SOU and Ryanair from BOH, although the LON airports are noticeably cheaper the fact that Flybe and Ryanair are able to charge these prices in the peak summer suggests that there is enough people in the area willing to pay more to fly from their local airport.