All of the compacted gravel roads are legal and lots of the byways are clearly marked, just stay away from any restricted areas and obey the flags and you’ll be ok.
If you go a bit off route don’t try to avoid or get away from the rangers, have a chat and tell them you thought you were on a byway and they will point you in the right direction. It’s the old rules, show them a bit of respect and you’ll be fine.
The plain dries really quickly but be careful of the chalk if it’s wet, it is very slippery (think wet ice). Also where it is very muddy or wet it can be very deep, perhaps up to 3 or 4 foot deep.