Ulwell Cottage is a nice site, but gets full in school holidays, we tend to use it off season.
You could pitch up on one of several sites along the Swanage heritage railway including Haycraft CC / camc / wotever (we did a long weekend there at the end of the easter holidays), the C&CC site at Corfe Castle, Norden Farm, Woodhyde Campsite, and others on this link:
http://www.ukcampsite.co.uk/sites/results3.asp?search=corfe%20castle&by=town
Using the railway negates the car park fees that soon mount up.
More around Swanage, here:
http://www.ukcampsite.co.uk/sites/results3.asp?search=swanage&by=town
including the well respected Toms Field.
For Bournemouth, these are available:
http://www.ukcampsite.co.uk/sites/results3.asp?search=bournemouth&by=town
Plus loads in the New Forest.
Brighton are here:
http://www.ukcampsite.co.uk/sites/results3.asp?search=brighton&by=town
ukcampsites is a great resource.