Its a Jaguar external fuel tank. There have been several instances of them being jettisoned over the years. The fins do break off on impact with the water- they have also been washed up occasionally. As for the length of it you can just see the ERU mounting shackles near the shingle. The oval just to the rear is the Fuel/Air connection.
There should be a warning stencil stating that the thing is made of or contains phenolic asbestos, which I think means don't set fire to it 'cos it gives off v. nasty fumes.
MM 111- you must be one big guy if your g/f can handle one of these.