Only reason I am wary is because surely you can't apply for the visa, successfully get it and then say to them 'oh thanks for granting me the right to work in the UK but can we hold onto it for six months?'. Because who knows when Qantas will call you, you know? What if you get the visa and then are holding onto it without using it for ages because Qantas UK aren't hiring? I wouldve thought that once you were granted the visa that you couldn't wait forever to use it. I thought I read somewhere that they could only postpone the start date by three months...