Since I have joined BA we have hired off the street to the 777/747/767. We now have ex midland FOs heading to the 787 and the fleet has gone relatively junior from within. The same can be said for the A380. Given we have only had the 787/380 for a few months not really much more to say.
You don't have to be senior in BA to fly long haul on any fleet. It's that simple.
Still one if the best flying jobs on earth.