The Garmin World Maps (GWM) are good for looking at a map routes, and it has a lot of roads that are not on the OSM maps, but you cannot create any routes on it. The best you can do is, as was said above, download the OSM maps.
The OS Maps don't have as many of the roads that the GWM has and you will find that there are some gaps on roads so the routing can sometimes be weird and take you a long way around. When that happens I switch to the GWM and look at the route there, find the gap then split the route at that point and do a track between them.
It's frustrating but at least then you can plan the route you want to do.
EDIT: I should add that I am taking about remote roads here not ones in built up areas, there you will be find with OSM.
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