A true wholeprey diet is one that includes the entirety of the prey animal including the skin, feathers/fur, organs, glands, blood, and intestinal tract. Advocates argue that it is the most natural way to feed since it most closely replicates what wild dogs and cats consume. Every component, including the less appetising ones (for us humans at least) serve a valuable function: Fur, feathers, and intestinal contents, for example, provide fibre which supports gut health; Internal organs, glands, and blood provide an excellent source of of vitamins, minerals and other micro-nutrients; Sinews and tendons provide mental stimulation and help to keep teeth clean and healthy.
The difficulty is that wholeprey meats are not considered safe for human consumption and therefore cannot legally be used in pet foods for sale in the UK. You instead have to source wholeprey meats through specialist suppliers and balance the diet yourself. There are plenty of good advice sites out there on the subject.
Clearly then, no store-bought food can ever really be wholeprey but some manufacturers have repurposed the term to describe foods where the ratio of meat, organs and bone used is representative of that present in the whole animal.