Join GLASS - that will give you access to TrailWise. Memory Map, Trailwise etc will allow you to purchase electronic OS maps but just because there is a byway marked, it doesn't mean that you can drive it.
Trailwise is the next best thing to the "Definative Map" - which your local Rights of Way Officer at our local council will use to definitively determine if a byway is drivable or not.
And yes, don't publish routes etc. on public sites as the info will get into the wrong hands.