Parkland Walk (starting in Highgate N6 5TJ) is a good off-road ex-railway line route with a gravel surface, but cycle.travel won't follow it (and nor will Cyclestreets, come to that.) I have "any" surface set.
I wonder what the problem is with this route. The surface is not perfect but as off-road routes go it is fairly typical. It is officially a shared route with pedestrian priority; this is the same as The Regent's Canal towpath, which all cycling apps seem to love, even though it is so narrow and busy with pedestrians that it is very slow for considerate cyclists.