I don’t think the CAA have a blanket ban on carrying stores, but it’s quite hard to do. I remember reading that the Sea Vixen was looking to operate with it’s drop tanks but that the clearance was going to take a while to satisfy the CAA.
Obviously most external stores are designed to fall off the aircraft at some point! I’m guessing the CAA require that to be virtually impossible, as well as ensuring that the aircraft is still operating in accordance with it’s airworthiness cetificate etc.