Got this reply from the Forest of Dean tourist board:
Thank you for your enquiry. Wild camping is not permitted in the Forest of Dean on Crown land, which is managed by the Forestry Commission. It is not permitted for various reasons, including the risk of fire, litter, risk of habitat damage.
If you would like to discuss this further, the Forestry Commission, who will be able to give you more concise reasons for the above, can be contacted directly on 01594 833057.
The Forest does cater for a diverse range of campers and therefore if you wish to have a look at our camping brochure, we would be happy to send you one if you let us have your address for postage.
Best wishes for your trip.