It does appear to be a British Pattern 1856/58 sword bayonet. There are four variations that I can make out and they all involve variations of the hilt (the mortise slot for the lug on the rifle and the top of the spine) . If you could post a better picture of the top edge of the hilt maybe we could come up with a better idea of which one it is.
Portugal has a similar one for the Westley Richards rifle, but the grip plates appear to have only two retaining rivets and the blood groove is different.