Usually mixes: AY (domestics operated by FlyBe and some HEL-LHR or LHR-HEL operated by BA, most by AY) on AY tickets. First time experiencing this trouble last week. BA operated AY seats have been difficult/impossible to change always. And the FlyBe operated AY seats have (since the start of FlyBe Nordic) been possible to select only at the time of online check-in.