There are 280,097 entries in this new database from Ancestry.co.uk containing master and mate certificates issued to merchant seamen by the British Board of Trade.
"The certificates and other documents included in this database were issued to merchant seamen who qualified as masters or mates aboard merchant ships. Records include certificates of competency, certificates of service, examination applications, and other documents. Forms vary, but they may include the following details:The description warns that "Some of the information is included on the back of the certificates, and some documents in a seaman’s folder have not been indexed, so when you find a record, use the arrows to browse surrounding documents to make sure you see all the records available. You may want to browse through an entire roll since the original folders were sometimes filed in random order."
history of service (dates, vessels, occupations, years in service)"
I searched for a couple of people I knew to be active during the period. One was not found. There were 38 entries for the other lots of multiple entries - on 16 people. While it's good to have image originals some of those entries were of a certificate with scant information. Others gave date and place of birth, a physical description, etc.