A small part of my ride today was through Forestry England land. So many ways we could have taken through this area that aren’t marked as even paths. Once you’re there on the ground it’s obvious where you can and can’t go. Even the footpaths marked to the top of the forest are actual double width tracks when you’re on them. Everything dotted in the map was easily accessible by bike and there were many more not marked that you could ride.
A combination of OSMaps with a subscription can be invaluable with route planning. Sometimes combined with a look on street view to see if the end of a non designated track is gated/signposted.
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Strava's heatmap feature an also be quite useful for finding trails that are off the beaten path; it takes data from anyone with their privacy set to public and aggregates it into a heatmap with white trails showing the most common routes, light blue being somewhere in the middle and then thin dark blue trails to signify the less common routes. Of course this doesn't account for any actual right of way so be wary, but many of my routes to this day are basically from creeping around Strava Maps/others activities and linking routes together so it's always worth giving a look.