As another poster pointed out EZY's largest operating base, a London airport, is just 80 miles up the road whilst EZY also have bases at two other London airports.
SOU's biggest selling point is it's direct rail link with London, it takes away passengers that may otherwise utilise London airports.
Unless an operator wishes to poach London passengers to a south coast airport, which surely EZY does not, then there would be little point in them operating there.
However if EZY wishes to catch passengers from the south coast from a south coast airport that offers appropriate facilities, particularly a runway length, then I'm sorry to suggest that such an airport would be BOH.