Bing Maps has free OS maps. I’ve found some great trails by pouring over the maps (not always on Strava incidentally)
Google Streetview and Satellite view can be useful to see if a track looks fun or just heads across a field 😀
Mapmyride or Garmin Connect to plot and work out roughly what amount of climbing there will be and how long the route will take, plus download on to my Garmin
Having a train station somewhere along the route is also a great idea!
It is also worth exploring when it is dry as you are less likely to get bogged down halfway down some nasty track in the middle of nowhere. Exploring down contours and climbing up roads or wider tracks helps here too. Let gravity be your friend 😆
We have so much technology at our fingertips now there isn’t really any excuse to not hunt out new trails 😀