I can confirm that records do exist for personnel who served before 31 March 1975.
In general, those relating to personnel who left the Army and Navy before the early 1920s are held at the National Archives where they are available to the Department and members of the public alike. Records of the first 329,000 RAF personnel who served in the Royal Flying Corps (RFC) before 1 April 1918, and in the RAF (formerly RFC and Royal Naval Air Service) from 1 April 1918 are also held by the National Archives. Records relating to personnel who served after that date, who left service before 31 March 1975 are held in the pan-Government archive at Swadlincote where they are available for departmental use.
These records contain details of postings, promotions and length of service but not normally of pay. Pay records were retained for around seven years after an individual's discharge at which point they were checked for accuracy and, if no credit or debit remained outstanding, they were destroyed.