International travel mostly works on a 1 in 1 out system where if Qantas is allowed to fly out of Aus to the UK then a UK airline must be allowed to fly to Aus with the same number of seats, hence why when V Aus went to the US Delta came back.
These deals are negotiated at a political level where they come up with the seat numbers and these seat numbers need to be shared with every airline based in Aus wanting to compete on the route.