Obvious
Why use a C17 to take a Merlin to Belize??? Why on earth would you want to waste tens of valuable aircraft flying a Merlin across the top of the atlantic to get all the way down the States to Belize (and back!!)! Use a C17 and go direct. Shame the C17 load-"master" was blind.
And think about it. Lots of Brit troops go to Belize to train in the jungle. Why not aircrew? They need Jungle training too!
And it wouldn't be a "new" Merlin. They are not even in service yet (if they ever will be).