you cant be going that fast on a fully rigid bike compared to a mtb so canti’s should be fine if i’m thinking right ?
You’re not thinking right, at least as far as ‘a network of roads and off-road trails in the Chilterns’ is concerned. You’ll be going faster.
Cantis are garbage nowadays by any objective measure. If you’re racing then you need them, because you need to give face to the heritage of the sport. Plus stopping the bike is not high on the agenda of most riders in a CX race. If you’re not racing then get a grown-up set of brakes. Particularly if you plan to take your CX bike on any serious MTB trails, which is fun to do every so often, then you definitely will appreciate disks.