@TXV24 With two freshwater turtles known from the Hamstead Beds at Bouldnor on the Isle of Wight. The trionychid turtle Trionyx and the testudinoid turtle Emys as far as I’m aware.
My best educated guess due to the location of the find and abundance of turtle material to be found with the right conditions at Bouldnor would be turtle.
Are you familiar with turtle anatomy and recognise such a bone.