I believe if flying out from LHR as a BA Silver on EI you can access the Aer Lingus lounge there. No free baggage or free seat selection on EI basic fares within Europe (which it sounds like you have if being asked to pay for seat selection) not even for elite Aerclub members so you are stuck on that one I'm afraid. Others may have more information for you regarding any additional benefits that I'm not aware of.