You have picked up a small nodule of Flint, which is a form of cryptocrystalline quartz,and occurs naturally in the Chalk (ie the Chiltern Hills in Oxfordshire). It formed soon after deposition of the Chalk (around 80-90 million years ago), a metre or more below the sea-floor. The silica was derived mainly from the dissolved skeletal elements of hard sponges, in the generally alkaline environment that existed below the sea floor.. When organic matter rots, it sometimes can create local acidic conditions. The silica can no longer remain in solution in these conditions and reprecipitates as a form of silica gel initially, while the chalk (calcium carbonate) dissolves instead. This process occurred preferentially in more permeable parts of the sediment, such as galleries of burrows (probably excavated by shrimps). Consequently, the most flints are replacements of the chalk in and around burrows, and the shape of a flint nodule generally reflects the original burrow morphology. Yours is a piece of one of these "Nodular Flints", and constitutes the majority of flint nodules which can grow even bigger into quite large irregular masses. Sometimes, they were so abundant that they coalesced into a more or less continuous bed, forming a so-called "Tabular Flint".
So, while this is not exactly a body fossil, it is the direct result of animal activity in the Cretaceous Period which has been preserved and is called a Trace Fossil.