EXCLUSIVE: Steve Regan, who oversees Big Brother for British broadcaster Channel 5, is on his way out of the Viacom-owned broadcaster. I understand that the company’s top entertainment commissioner is leaving the network and has been telling top-level execs in the UK. A number of producers at the recent Mip TV market in Cannes told me that Regan is no longer at Viacom's Camden HQ. Regan is one of the most senior entertainment buyers in the UK, having overseen Endemol…