This is the unfused distal articulation of a bovid or cervid metapodial.The metapodials are the equivalent of some of the bones in our hands and feet, although in the cow and deer family (bovids and cervids) these are fused together and function as part of the leg.
Sorry for the massive link but there is a good picture here that explains what part of the body this is (in this picture it is called the cannon bone):
I can't tell from this if it is from the front or the back legs (a metacarpal or metatarsal). The lower (distal) end of the bone was unfused to the rest of the long bone, suggesting that this came from a young animal as the ends of the bones fuse with age.
I can't be absolutely sure from photos but it is almost certainly not a fossil but a recent bone. Bone can become somewhat mineralised and coloured like this quite quickly in water. I can't tell what species it came from, but the probabilities are hugely in favour of domestic cow. Huge numbers of cows have been farmed in the UK, making their bones the most common of all bones finds and they end up in rivers, beaches, gardens, everywhere really!
I hope this is of interest, please do ask if you have any more questions.