I use strava route planner, just because it’s dead straightforward to use, however, I’ll study an os map first (either online or a paper version) to familiarise myself with the area I’m planning on riding, so you can suss out about in the area (fire roads, farms, pubs, car parks, café stops etc) as well as checking out the terrain (moorland, bogs, sink holes) and what you can *ehem* legally ride on.
If you have a PC or small laptop then it’s handy to have the Bing OS map (cos that’s free) open on one screen, or on your phone and then your Strava route planner open on another screen, then you can plot your route better that way.