We went to Oz last year on the cheapest Emirates flight we could get, we thought no alterations were allowed, but when I fell ill and we had to postpone our return, we were allowed one change free - then it would have been £25 for any subsequent changes. Suggest you book with a return date well in the future without mentioning in advance that you might want to change it. Don't forget UK nationals need visas.
Best of luck.
UFO