Hi there! Welcome to OpenStreetMap, and thanks for your contributions!

Quick Q: did you check whether these roads are covered by a No Pedestrians sign? I've not been on the ground in that area for ages, so I don't remember.

"foot=discouraged" is better if there isn't one of those signs. Because.. well, "foot=no" is for local legal prohibitions, and we're talking about UK law here, where walkers have the enshrined legal right to use any road other than motorways roads or slip roads, unless there's one of those signs.

Pedestrians are still widely discouraged from using dual carriageways or busy dualled roundabouts though, and foot=discouraged absolutely fits the bill out here for the main motor carriageways.

Routing engines are supposed to foot=discouraged the absolute last choice, or no option at all.
https://wiki.openstreetmap.org/wiki/Key:access#access-discouraged has more. I think it absolutely is a safety issue, and access=discouraged works well to tag that stance if there's no pedestrian-in-a-red-circle sign up.