@HobbesOnTour myself and a couple of friends are planning to do a lot of gravel riding this summer. We've been looking at planners and decided Komoot would be the best option.
I haven't looked at cycle.travel yet. I wondered if you feel it would be good for this sort of planning?
I don't know is the short answer!

Sorry! Not terribly helpful!
I think different planners may be better in different areas and possibly for surfaces, but I have no evidence for that, only a feeling. I do know Komoot will throw me onto any old surface even in roadbike mode.
I have only deliberately sought out off-road routes twice, in order to test out a trailer. Both times I looked for routes close to me on RWGPS (I'm not even sure if I can do that now without a sub). All I wanted was rough terrain, I didn't care about anything else.
As a general rule, I'm not bothered if a route is all paved or not. My bike will go pretty much anywhere.
Two things concern me based on experience - will a route bring me to a dead end or put me on a road that no bike should be on?
Komoot has done both to me, albeit in places quite far from where you are. Cycle.travel? None that I can recall.
Cycle.travel has a paved/unpaved toggle so I see no reason why it wouldn't work. Off-road, Google Street View won't work, though, so if that's your thing you'll need another source to have a good peek.
Of course, your question is better asked of
@Richard Fairhurst
There are possible legal issues, I believe, with bike access on some paths, as well as practical issues where farmers, for example, have placed fences/gates. Not a problem if you're walking, a bigger issue with a bike. I'm not sure if Komoot has as good a handle of the British path labelling as cycle.travel.
I'l also aware of issues in NL & Germany with cyclists using traditionally hiking routes and causing friction.
As with any planner in places that you don't know it can be a risk. Alternatives may be fewer or non-existant in out of the way places if you hit a dead end. Then, a cool group of friends is important
Sorry! Not a lot of help!
You could try "test routes" comparing one to the other and get a feel if one suggests routes more to your preference than the other. (That's what I've done).
Also, Google is your friend. People post gps files of their routes all the time. Or Youtube!
At the risk of stating the bleedin' obvious, it's entirely possible Komoot will create the best route here, and cycle.travel there!
Good luck!