The XLR won't be available for JB's London start date (approximate start date range, as you know). That was known when they placed the order for the LR and made the London announcement. The LR has more than enough range for London, all year.
They will almost certainly place an XLR order, at some point, for destinations beyond London/Dublin.
JB will operate a mixed fleet, with LRs doing London/Dublin (MAYBE Paris/Amsterdam) and eventually XLRs to Paris/Amsterdam and possibly points beyond.
This is my opinion of course, and I suggested as much many months ago. We'll see if I'm correct.