Doesn't JER get ET/CE cabins hence counting as Europe for BA purposes? It counts as domestic for immigration.
You're right on the cabin level. I was thinking of how I've used it to position for long-haul CW redemption, saving the APD, as I have to be in St Helier frequently for biz, so I do it the day before my London to wherever Avios flight. Then you get a free transfer (even under the new regime), like you would from any domestic starting point.