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.
It’s been tagged in OpenStreetMap as smoothness=bad, which roughly means “not suitable for city or road bikes”: https://www.openstreetmap.org/way/938155966. cycle.travel tries not to route over paths/roads like that (and maybe Cyclestreets is similar). I suspect the tagging might be a bit extreme in this case.
Yes, tagging is extreme, I think. I use Parklands Walk all the time with touring or mountain bike. The surface is pretty good in all weathers and it is a brilliant traffic free route which is hill-free (a very slight incline in the westerly direction which is nice to descend on return). It would be fine for a commuting bike. Cycle.travel will snap to it for planning purposes.