Well the ten pounds still exists either way. You'd have to argue that there's more utility in Bentham owning the £10 than the mugger owning the £10, and that the difference in utility between them is greater than the utility of a finger.

I imagine you could define utility that way, but presumably the mugger could increase the cost (two fingers? an arm?) until the argument works. Also, if you do definite a utility function like that (say, "there is more utility in this £10 being mine rather than yours than the utility of your arm") then that's a pretty questionable morality.