Equipment
To start with you don't need to get any special equipment, but there are several things you will probably have at home and should bring with you, see the list below:-
1) Remember this is Britain, so the weather can vary dramatically, just because it's dry and sunny when you leave home doesn't mean it's going to stay like that, so always bring some waterproofs and pleanty of warm clothing, even on sunny days it can be pretty cold in the middle of a forest stage.
2) It is also important to bring plenty of food (even if the event supplies a packed lunch, as they are very small) also hot drinks, as this will help beat off the cold.
3) Good strong boots or shoes, as the ground will probably be rough and uneven.
There are a few other items which are not essential but are very useful, and worth bringing if you have them, a Hat as this will prevent heat loss in the winter and help prevent sun stroke (assuming we get any sun) in the summer, a whistle for warning of on coming cars, a fluorescent jacket, to make it easier for drivers to see you, a pair of wellington boots can be useful if its very wet.