There's nowhere like Salisbury that has the level of access. There are some good routes around Nidderdale, Yorkshire way but have you considered just riding some of the local small back roads, around here they're as tricky as any graded gravel road with constantly changing conditions. I wouldn't bother with the forest drives unless you're in the area anyway as they're pretty easy going.
Many high moorland routes are not to tricky, if you can see twin tracks on google satelite view it will most likely be what you're looking for.
Many high moorland routes are not to tricky, if you can see twin tracks on google satelite view it will most likely be what you're looking for.