We just got back from Afghan (1RHA). If your going to join Kings Troop get ready to work harder than you've ever worked in your life mate, shovelling horse shit everyday ...eugh. Join RHA , way forward for sure : )
UP THE EAGLES!!!! (That picture is a depiction of N Battery (The Eagle Troop) earning their honour title).
NO you can't be Gunner Log in King's Troop (I think). KT is a ceremonial unit, but individuals from within can and do get attached to other Gunner units to serve a tour. At Ph1, as a KT volunteer (that's what you'll be) you'll be given the trade of mounted Gunner. (cue smutty comments)