No agreement required
The CAA has no jurisdiction over what professional associations or institutions its licence holders choose to join. If a professional association or institution secures an advantage for its members on the basis of the members holding a licence again it has nothing to do with the CAA. E.G a pilots union negotiating preferential life insurance for its members on the basis of the occupation group requires no agreement from the CAA. If the professional institute gains international validations of UK Licences for its members it has nothing to do at all with the CAA. I am sure that EASA would welcome the opportunity for continued co-operation and harmonisation with as wide a group of pilots as possible. In due course if the Professional Institute gains recognition by EASA as a competent authority then individual pilots might rely on their professional membership of the institute for their on going licensing while some might also choose to keep a CAA licence active in the background.