Looks like Strange isn't kidding when he says it's complicated.

“All of English magic?” Rita echoes, a puzzled look on her face. “How do you... what does that even mean?” Is he referring to the quantity of magic energy, or something else...?

For a moment, there was a glimmer of hope that Strange had found a way to definitively defeat the fae, but that's already been crushed under a wave of doubt that it's anything resembling a repeatable process. Besides, if it was, Strange would probably be gloating a lot more.