In reply to Kemics:
If you are going in August then try to get into Eweleaze Farm (just East of Weymouth) -
http://eweleaze.co.uk/ a big site, but has some quieter areas and a friendly vibe. They allow open fires and the site has its own excellent beach (with views across to Portland). It does get booked up though and only opens for 4 weeks a year.
2017 prices:
Adults
£8/night Sunday to Thursday nights, and
£16/night for Friday and Saturday nights (and Sunday of the bank holiday weekend)
Vehicles
Flat fee of £10 for each vehicle brought onto site for the stay