Intrigued by the beautiful pattern of these feathers, I consulted a field guide* and found the most likely former owner in the families of falcons or woodpeckers.
In the former case, the underside of a peregrine would be a good guess and alternatively, these feathers could have originated from the back of a lesser spotted woodpecker.
I find that, more often than not, curiosity is more enjoyable than it is important to find the answer! It gets me to look, see, hear, feel, smell, read, experience…simply learn things by accident, not determination.
*Peterson R, Montfort G & Hollom PAD. 1954. A Field Guide to the Birds of Britain and Europe. Collins, London.