Just had this from O2 on twitter after i asked about switching to a nano sim and buying the phone outright.
O2: @zimpaz Hi James, the Apple store or O2 store will be able to provide with the correct sim at the time of your purchase
(44 seconds ago from PulsarEngage in reply to zimpaz)
O2: @zimpaz Hi James, the Apple store or O2 store will be able to provide with the correct sim at the time of your purchase