Graham K
Out of the airports you ran through only bournemouth offers low cost travel at present. Ryanair operate two routes and are planning to add another in 2003. Rumours ripple through the airport about other possible carriers planning to start low cost routes.
Southampton was rumoured to have Go sniffing around, but now the merger with Easy may put an end to that option as Easy have a base up the road at Gatwick.
Exeter is a little close to Bristol for Go/Easy and would clip Bournemouths catchment to the west for Ryanair. Putting a base in Cornwall really would be unfeasible population wise.
Only Bournemouth and Southampton are well placed in the Central South Coast area to generate the PAX nos required. Southampton has a nice modern terminal, but Bournemouth has one in the pipeline (planning now granted) and regarding the length of Southamptons runway. Its big enough to land fully laden B738 from Tenerife in the winter.