CILIP is the "leading body representing the information professions" in the United Kingdom. They offer certification, chartership and fellowship. Is membership of this organisation, and similar ones, useful for career progression?
Yes, membership in professional organisations is often beneficial, both to prospective employers and in terms of networking in the industry. The people you meet and interact with in these can be directly beneficial to your career. You also get an opportunity to showcase your talents and skills to an appreciative audience at times.
Furthermore, there is no drawback, belonging to a professional group is extremely unlikely to impact negatively on your career.