@Herr:
But according to the other rule, Yugoslavia “is no longer considered neutral and immediately becomes part of the alliance opposing the power that attacked it”. So if Germany attacks Yugoslavia and then retreats from it, that rule would imply that Yugoslavia is no longer a neutral (and thus, also no longer a friendly neutral), but a full member of the Allies. So at the start of the British turn, it would be friendly, and not friendly neutral.
You are completely right. For any reason I misread your example mixing in parts of the first posting that was discussed here in this thread.
When it comes to the British turn, Yugoslavia is an Ally, because of the German attack before, so the territory of Yugoslavia is friendly to the British.
So in NCM the British fighters may land there.
Sorry for the confusion and thank you for pointing it out. :-)