cycle.travel prefers paved surfaces but will route via (good) unpaved surfaces if it'd save a long detour or avoid a busy road. It highlights unpaved sections i

cycle.travel prefers paved surfaces but will route via (good) unpaved surfaces if it'd save a long detour or avoid a busy road. It highlights unpaved sections in green so you can easily drag the route away. Alternatively, bikeroutetoaster.com has a paved-only routing option, and maybe gpxeditor.co.uk too, though the sites can be a bit flaky.