I experienced this when flying EDI-WAW. Cityhopper you can choose any seat with Elite status, but other intra-European flights you can't. Try checking in for the AMS-BCN flight in Newcastle I was told it's because they don't know where the curtain is going to be, so they can't assign you a seat. However, they did say that you automatically get a seat assigned to the front with Elite status –in my experience this doesn't happen...