On P&O you can only choose your cabin on a select booking, the saver or early saver are both guarantee, where you can choose your location but not the actual cabin.
On a select booking when choosing your cabin type, scroll down till you see choose your cabin, it will then allow you to choose your location and then it will show any different prices for each deck, choose the deck then it will select a cabin for you, if you then press the down button against that selection it should take you to the available cabins in that area.
It is far more slow and clunky process than Princess but unless you are seeing only a cut down version, then I hope the above helps you.