I do wonder if it's a case of him not being 'officially' told but the British knowing in advance that if the U.S had the chance they would take it. I think it's better for him that, whatever the truth of the matter is, he can say he was not told in advance. It reduces our complicity and gives us more options in dealing with Iran than we otherwise would have.
Let's not forget that
Nazanin Zaghari-Ratcliffe is still in an Iranian prison and if we were viewed by Iran to have participated in this attack then she might have the consequences for that. Although again I wonder how much Iran will believe that this was nothing to do with us....