Not sure why you would want to fly into Stansted airport given the lack of flights there. It depends what airline you were hoping to fly. Off the top of my head, the list of Qantas partnered airlines that fly into Stansted are few and far between. Basically just Emirates.
If you are looking for Emirates classic rewards flights availability, calling up might just be the most simple option. The website is a bit stubborn and depending on the agent you get on the phone, even they might not fully understand. If you have a tool like expert flyer, that lets you see the availability of fare classes, you can also just look up the availability for the flight you want. Keep in mind though, Emirates only seems to load this flight into their schedule from March 31 onwards.