i can't believe you've done this
Something uttered by British people after they've been suckered punched.  Usually preceded by the exclamation "Aw fuck", and accompanied by a look of complete bewilderment.
British guy: Blah Blah Blah Blah
British guy's friend:  *punches British guy in the face*
British guy:  "Aw FUCK, I can't believe you've done this!"