These events tread a fine line legally. The rules start by saying riders are totally self reliant and responsible for themselves, and then go on to point out what you must do and take. They insist on third party liability insurance, but presumably don't have event insurance.
I'm not criticising, but pointing out (again) that some organiser will someday get rinsed in court
Forestry England now charge a % of entry fee for events, but I'm guessing won't be made aware of anyone planning to ride through their land - usually not an issue, but for a paid event.....?
Once events are organised officially (insurance, medics etc) costs very quickly escalate.
Looks a good excuse for an adventure though.
If you do it Shaf, look for the off-road/byway heading NE from near the CP at Bewcastle. It goes up past Black Preston/White Preston and will take you straight through Kielder Forest, past Churnsike and on towards Bellingham, and onwards to Bamburgh. I'm tired just thinking about that stretch.......