Britain as a whole, including the RAF, did not use the metric system at that time; it didn't start seriously moving towards metrication until the 1960s (see e.g. Tony Benn's statement of 1968: "In May 1965, the Government announced their support for the adoption of the metric system of weights and measures in industry which had been proposed by the Federation of British Industries. They also accepted that the metric system would spread outwards from industry and become in time the primary system for the country as a whole. The Government consider that this will bring substantial advantages.")

Looking at a page from the Lancaster's pilot's notes you can see that altitude is given in feet, fuel capacity in gallons, speed (IAS - Indicated Air Speed) in miles per hour, and boost pressure in pounds per square inch. There were oddities, instruments and pilot's notes used centigrade (a now-obsolete term for Celsius) for temperatures; machine gun calibres were generally in inches (the Browning .303 and .5" being the main weapons) but cannon in millimetres (the primary weapon being a licensed version of the European Hispano-Suiza HS.404 20mm autocannon). For the most part, though, Imperial measurements were used, including pounds for bomb weights.