While possible export customers settled down to watch the progress of the programme before expressing any real interest (and soon losing any they had), by mid-1956 over 20 FAW.Mk.1s were engaged in trials work - the type of development batch arrangement later formalised for production of later types such as the Lightning. Despite the designation of FAW.1, none of these aircraft were ready for real service - while 46 and later 87 Squadrons played with their new toys, they were covered by many limitations on the flight envelope and they were not missile armed.

In November 1956 the FAW.7, the first mark to actually carry the four missiles specified in the original requirement, first flew. Basically an FAW.5 but with uprated Sapphire Sa.7 engines and powered rudder and extended rear fuselage. By this time so many different marks of the Javelin were in the air it was a wonder anybody had any idea what was happening. A larger number of each mark were being used in various trials on such basic items as the weapons and engine fits, leading one to believe the Air Ministry had handed the RAF over to Gloster as one big Guinea Pig. The FAW.8, an FAW.7 with reheat, appeared so quickly that a full 80 FAW.7s never even saw service, being delivered straight into storage at RAF Kemble, and later converted to FAW.9s.
It was not until June 1960 that an RAF Javelin - an FAW.7 - finally fired a Firestreak missile, successfully downing a Meteor drone. The reheat-capable FAW.8 was limited to using reheat only at a minimum altitude; below that point engaging the reheat actually caused a loss of thrust (to the point where take-off could not be safely accomplished with reheat engaged).