Originally Posted by
ThrowingBoeing
And if I was to fly in Y I could use the terraces lounge at MAN by virtue of status?
You would have a ticket to fly on BA1371 MAN-LHR and you are a BA Gold. For the purposes of lounge access, it really doesn't matter whether the fare on your ticket was filed by BA, CX, QF, QR, VS, AC or UA, nor which one of them issued the ticket.