Not sure what you mean by central road? If you mean the main one N from the Bustard then that is always open when red flags are not flying as its a BOAT with no TRO.
The link you posted is for the existing network (as shown on current OS maps). Some of the routes will change when this comes into force. See maps 3 and 4 on the MOD site.
If you are unsure of any aspects of legal laning / Salisbury Plain then I recommend you hook up with a club/group/individual who does.
I did it the "hard" way and did it myself via the online county council maps/ council TRO list / paper OS maps / RoW officer contacts to be 100% sure in my mind I was legal and safe (beware UXB!!! ). If you mess about up on the Plain you will be caught / reported by others and potentially have your motor taken away.for good!
That said, on the ground up there there are a multitude of tank tracks and BOATs so navigating is not the easiest task. I invested in a Memory Map 2800 backed up by paper maps etc so I know I'm on the "right" track....
HTH
Rob