There's very few things you can do with a forest, without changing the planning use on the land (which is difficult, but does occasionally happen). Basically, you'll get a licence to fell a proportion of the trees, which can be sold as timber but the forest needs restocking. This might be naturally done, in which case costs would be low. Also bear in mind, hardwood marketing is very variable. The main other use would be sporting rights - but if its previously Forestry Commission area, then its likely a condition of public access is there, meaning that you can't do certain other activities.
The 4x4 angle is interesting - at a low usage, I think its less than 12 days per year, you don't need permission. More than that, and you'd need planning permission for change of use (probably say no), environmental impact assessment (won't be great), etc etc. Also don't forget things like public liability insurance, etc.