If you own a boat you may wish to undertake RYA training to keep it and you safe, and to affect insurance. If on the other hand you plan to charter then companies will require evidence of practical training to protect their boat, and their (don't forget your own) Insurance.
Anyone taking a British flagged vessel or chartering in the Med will require an ICC (International Certificate of Competence ICC), which is based on a Day Skipper practical Course Completion Certificate or greater.  In the UK it is up to individual charter companies as to what certificates they require clients to hold. In all cases ask the RYA training instructor for the applicable RYA Course Completion Certificate. Charter companies may additionally require less formal evidence of experience for other members of the crew.
Note the ICC or CCC certificates are quite different from the RYA/MCA Yachtmaster Coastal, Yachtmaster Offshore, or Yachtmaster Offshore Certificates of Competence for which you are more rigorously examined.