I was born 1968. Sometime around 1978 I read a story about some kids who set out on adventures on their “ponies” and they would stop and camp in the woods and eat “tinned“ food. I believe it had a very UK British feel. They would talk about riding on the “moors” and through forests.
It is possible they were trying to solve a mystery, or just narrating their experiences along the way as they traveled to an ultimate destination- like maybe a beach or shore as on a summer adventure.

I will stop here because I do not want to confuse this with another story in my head.

I think this book might have been a hard cover, about an inch thick, with black paper on the cover inside the dust jacket, and MIGHT have had gold lettering embossed “ONYX” on the cover. MIGHT CONTAIN “nebula” That description might also pertain to a different book but, that is why I am here. They seemed to make a big deal about calling the horses ponies and they all seemed very independent to be traveling without adults. Approx character age 10-15?
I read the story when I was about 10 years old, around 1978, so there’s no telling how much older the story actually is?

I’d really like to get another copy of the story of those children traveling with and riding their ponies, camping or resting while eating food from tins and parcels, and traveling across the moors and forests to get to…?