I Am not an expert but it could a spine, made up of a few vertebrae. It could be off a plesiosaur, but I think Surrey has land based dinosaur fossils so it could be off a megalosaurus. If it is a megalosaurus spine it is rare, but it also could be a large piece of an ammonite. There is about a 10 percent chance of it being a spine and a 90 percent chance of it being an ammonite. If it's of an ammonite it's a very large specemin.
It is the outer whorle of a lower cretaceous hetromorph ammonite called Tropaeum bowerbanki.
It is from the lower greensand - ferruginous sands, Isle of Wight although you can find them in other areas around the U.K it looks about the same size and preservation as the ones we have collected on the island.
Please see below an image of a similar ammonite to yours, we found this on the I,W many years ago but it is poorly preserved.
Because it was our first big ammonite we have kept it.