Inside this plantation is a feature not observed by many.
It hides beneath the tree cover and may be associated with the similar feature seen about 500 metres south.
Whilst its purpose may be unclear, it appears as a woodland-designer boundary on this historic map
Link
The other two may, apparently, be ground-markers for aircraft training for bombing missions, they are all but invisible close-to.
This one, however, remains intact and traceable for all its shape - see
NN8403 : Dark secret revealed