2 minutes in episode 1, we see the butt of Hugh Bonneville, as well as the butt of Mathew David.
12 minutes in episode 1, we see the penis of James Faulkner . He is having a bath with Matthew Tennyson. We can see his uncut penis in the water.
Towards the end of the episode, we also briefly see Tom Riley's butt.
7 minutes in episode 2, we see a partial view of Elliot Cowan's butt.
35 minutes in episode 4, we get a good view at Tom Riley's bubble butt.