Or else:
(1) The British have all the military options available to Americans. We have everything from nukes to the SAS. We can do military stuff. That's one answer to "or else what?" That's why you wanted us as allies.
(2) Apparently you Americans love us Limeys and fully support us. You said so in your post. And you are the world's only super power. That also is, surely, an answer to the question "or else what?" As we are your allies, surely you are on our side? "Or else what?" Or else, at least, the US will back us up, rather than saying "or else what?"
We are your allies. Honor requires that you guys should side with us against Iran, rather than sitting on the sidelines and asking us what we're going to do about it. If you won't, we shall draw our own conclusions.