I've heard many aircraft declaring minimum fuel, but as RAT has said, do not expect any special treatment until you state your current endurance or declare an emergency. As for service, I do know of a colleague who declared a "Mayday" at LHR and he was given an approach immediately. I think this will be the case everywhere. The annoying bit is that you have to play this "game" until you really believe you will get to your final reserve.