To fans of this and other British crime shows - current and classic - I'd recommend BritBox. It's a collaboration between the BBC and ITV and has some real "mystery" gems [1]. I've no affiliation.
Let's see, can I get it from iTunes? Nope, Apple only sells 7-13 in the U.S. too.
(Still waiting for the video rights holders to learn the same lesson the music rights holders eventually did: if you don't want people to pirate, make it easier to buy it than to steal it.)