Barcelona should be a success from most U.K. airports including Southampton but the fact that Vueling was a totally unknown name to many at the time it operated it wasn't too much of a surprise that it was shelved and Flybe continuing the route a year later with a ridiculous schedule that too was doomed but with the likes of EasyJet flying SOU to BCN I personally think that the route could be a success, the same applies in a similar way to Nice.
Barcelona is my favourite city and one I visit a couple of times a year so personally I would love to see SOU to BCN return someday.