Keep in mind that to requalify for Silver, you'll need 4 flights operated by BA, however, if you go LHR-ZRH, ZRH-LHR, LHR-JFK, and ZRH-LHR coming back, that will be sufficient enough so that you can fly QF/CX on teh other available legs, minimize AA flights, and get silver with teh best service possible (this is subjective but I find CX>QF>BA in C unless it is a night flight).