ego.istic.org

This is a bit of a weird expression. It seems that until very recently, nobody I conversed with regularly used it at all. But now, everyone is saying it. At work, I hear dialogues like,

“We’ve got a guaranteed million pounds of profit this quarter.”
“Well, you say that, but some of those listed deals are still to go through budget approval.”

The expression indicates that I’m about to disagree with what you’ve just said. The “well” appears to be almost mandatory: some silliness with Google gives 75,600 results for the phrase “well you say that but” but only 30,900 for “you say that but”. How that works I don't know, but it gets stranger than that.

Among my group of friends atm, the phrase is used as a stand-alone utterance, as a sort of standard follow-up to when someone has made a trivially false, possibly ironic statement.

“Obviously, I am far more manly than all of you.”
“Well, you say that, but.”

It is always said with a deliberate, very particular inflection, and with nothing at the end of the sentence to indicate an ellipsis. The ellipsis is there in the meaning, of course: the utterance is notionally followed by the obvious objection to the previous statement. The fact that the ellipsis is unspoken may be quite significant. It indicates that the phrase isn't just being used as an abbreviation, but itself has turned into an accepted idiomatic statement equivalent to, “I make the obvious objection,” but with a more conversational flavour.