Depends which bit of southern England you mean ... south-east or south-west? Broadly speaking the further west you go from London the nicer things get (countryside + towns). [The south-east corner was nice about 30 years ago, but these days is incredibly densely populated by people with a lot of money and an attitude to match].
The South Downs are nice (just north-east of Portsmouth), as is the New Forest (SW of Southampton), and the whole south coast west of ~Portsmouth is lovely if you avoid the ports. If you can get as far west as Devon/Cornwall it's a real treat. These places will be packed with people at this time of year though.
Personally I try to avoid towns, but places like Oxford, Cambridge, Canterbury, Winchester are worth a visit once as are many of the other older towns. Places to avoid: Luton, Milton Keynes, Swindon, most towns south and east of London, and all of Essex!
Of course if you're looking for night-life and daytime tourism around one of the world's greatest cities, you don't need to leave central London!