I would guess September of 2020 at the earliest. They don't currently have a plane in service that can fly the route. This will likely be the announcement that they are taking the option to convert some of their A321's to A321LR's along with a route announcement. They exercise the options and take delivery a year or so after that? They'll likely run the plane on some short routes and then TCON routes to familiarize the crew and then a first flight to LON some months after that. (Which I will hopefully be on!). This is my totally uneducated guess.