Yup MAHOOSIVE variation, unless you're in France, then they are all pretty good.
Try this for reviews and stuff, http://www.ukcampsite.co.uk/ , its the TripAdvisor of the tent world.
I use this site as well.Still had a few dodgy stays at some sites but as long as they have a hot shower and I have paid less than a tenner then it will do for the night.
Any holiday site is likely to be expensive, noisy and unsuitable.