Rollo is correst - I guess about 1972 the school leaving age was raised from 15 to 16 and the CSE examination was introduced to enable less academic kids to get paper qualifications.
In 1988 the GCE and CSE were amalgamated into the GCSE that we have now.
But I don't know that it has ever been compulsory to take any exam. It is compulsory for schools to offer every child equality of opportunity, but regrettably the pupils can no longer be strapped to a seat and forced to get cleverer.
To check on results you need to lnow the exam board that provided them: try this link
http://www.qca.org.uk/qca_4521.aspx
Lastly, depending on the circumstances, this may be something of a dead end as qualifications gained beyond a certain period back tend to get disregarded.