In most cases no formal qualifications are required. However, many skippers will prefer to take crew who have undergone training to at least RYA Day Skipper level.
If you look for work through a crew placement agency, they will often recommend that you hold an RYA Day Skipper certificate, preferably supported by training such as the RYA Diesel Engine and RYA Radar courses.
You may need an MCA Yacht Rating Certificate as a certain number of crew on board are required to be qualified. For this you should hold the RYA Day Skipper or RYA Competent Crew certificates. You will also need additional STCW training in sea survival, fire fighting and prevention, first-aid, personal safety and social responsibilities.
Full details of Yacht Rating training can be found in the Guidance & Regulations section of the MCA website.