A good starting point would be the list of possible destinations from Southampton:
http://www.southamptonairport.com/flight-information/destinations-and-airlines
As the majority of services are with FlyBe, this website would seem to be the next one to consult:
http://www.flybe.com/
However your dates then start to cause problems. 6th September is a Friday but (for example) flights to Verona only operate on Saturdays and Wednesdays.
Nice is a possibility but not cheap. Out at 1600 on Friday for £142.65, return at 2055 on Tuesday for £76.39, making a total of about £219 per person (with transfers accommodation to be added onto that).
http://en.nicetourisme.com/
My own choice would be Paris (which is usually very pleasant in early September). You've got a choice of 3 flights on each day, with return fares starting from around £148 per person.
http://en.parisinfo.com/
Chris