There a good site at the edge of Epping Forest. It's run by the council and allows dogs, according to this blurb.
I've stayed there a couple of times and it's really good. Plenty of room and literally on the edge of the forest. It can get a bit crowded at weekends in summer though.
One thing, as I remember, you have to negotiate a bit of red tape beforehand so I advise ringing them in advance.
To protect yourself, you must protect everything that is not yourself.