You should be able to ride Byways (marked as red + + + on OS 50k maps) and ORPAS (marked as red . . . on OS 50k maps), but there are other lanes that you should be able to ride as well which are unclear in terms of status on the map. If you don't do proper research, you run the risk of either missing some good trails (at best) or getting into legal difficulties (at worst).
1) Use the Trailwise website
2) Join the TRF and get advice from local members, as well as enhanced access to the Trailwise website
3) Go down to the council offices and spend days examining the definitive maps and the lists of streets.
Not all ORPA will have rights for MPV - it depends on the history of the highway authority, although I only know of one that has no definitive map, thus publishes all public fp & bw as ORPA. Logically if it wasn't a vehicular highway it would be on the DM&S. Stick to BOATs and roads shown on the List of Streets. Your group RoW Officer will know more.