You can now access service files for the Corps of Royal Electrical and Mechanical Engineers regiment transferred to the UK National Archives from the Ministery of Defence. They are individually catalogued online in WO 420.
For other ranks, the catalogue entry includes name, usually initials and surname, service number and date of birth. If the person was born more than 115 years ago the file may be viewed at Kew. Those for a date of birth more recent than 1906 (at present) will be made available over time year-by-year.
Information in the full file typically consists of personal information and interactions between the individual and the part of the armed forces they served with such as record of service, disciplinary and conduct sheets, service postings and information captured when they first signed up.
Find out more here.