The episodes are now up on both US and UK iTunes. Links:
US Enemy: https://itunes.apple.com/us/tv-seaso...ld/id704921951
UK Enemy: https://itunes.apple.com/gb/tv-seaso...ld/id704921951
UK Web: https://itunes.apple.com/gb/tv-seaso...ar/id704945256
U.S. prices are $1.99 per episode or $9.99 per serial. UK are £1.89 and £9.99 respectively.
Mine are downloading now. Haven't bought from iTunes in so long I had to update my billing info, which felt like torture.
There's a trailer included with Enemy, which confirms that iTunes will be the only download platform for these episodes.