"You can’t contest a general election without knowing who will be Defence minister if you win…"
Actually you can - people don't vote for a list of Ministers but for a list of policies. As you're never certain who will be elected there are often changes to the slate after you've made the trip to Buck House and been photographed on the front step in Downing Street. Then you sit down and decide who does what
Any way Defence is so low on voters list of priorities they couldn't care less who is running it