I. O.
Take the M11 Motorway from London and follow the signs to Stansted Airport. Duxford is on your left before you reach Stansted.
One point to bear in mind regarding the use of 303 calibre machine guns at the design stage of WW2 British aircraft is that 303 calibre was THE standard throughout the British Empire. That means not just the UK and the Dominions but India (including what is now Pakistan) Malaya & Burma, as well as the Colonies in Africa and the Middle East and those States under British protection elsewhere in the World. The introduction of a new calibre of weapon in peacetime had enormous implications throughout the entire Defence Establishment.
Incidentaly 20mm cannon were used by 19 Sqn during the Battle of Britain but were withdrawn due to teething problems with ammunition feed.