Clifton Cruisers at rugby are my personal choice, very nice boats that dont scream "HIRE BOAT", good value, and a very helpfull friendly service. Not that close to the south coast but worth the trip IMO, i am also on the south coast and travel up there twice a year for our hols! also you have a good choice of routes from rugby, and no i am not related
Edited to add, you should also do some reading on boating before you go, i.e. locks, mooring ect, The Boaters Giude is very basic, but a good place to start, available on line as a pdf. Google should find it (i have not yet worked out how to provide links )