LEO stands for Lyons Electronic Office.
The LEO I computer was the first computer used for commercial business applications in 1954. It was a 1st generation computer using Valves. Two models later and in 1961 the LEO III was introduced. The LEO III was a 2nd generation computer, using Solid State Transistors and Diodes. in 1963 LEO was merged with the English Electric Company (who had built the EE Deuce). The LEO III was still produced for a few years.
The LEO III computers allowed concurrent running of up to 12 application programs through the main operating system. The General Post Office (British Telecom) ran LEO III computers up until 1981.
The LEO Computers Society have a very good photo page on the LEO III.