Ad-supported US streaming service Pluto TV made its European debut this week with a deployment at UK operator Sky, almost one year after sort-of competitor Zone TV teased us about breaking into Europe. It transpired soon after that Zone TV had won Liberty Global’s Virgin Media account in the UK, so Pluto TV’s arrival at a tier 1 rival marks any interestingly parallel route in, yet the two bring very different benefits to pay TV operators. So what’s the appeal of these US start-ups to the two UK majors? But more importantly, how does Pluto TV fit into Comcast’s plans for its new toy over the other side of the Atlantic? Pluto TV, founded in 2013, is landing on Sky’s…