got an answer
i spoke to an ex raf fireman today and got an answer. basically the raf and mod work on a quicker response time than the caa. under the military flying regs the crash crew must be at the scene and have started to rescue people in under 3 minutes. i believe that the caa state only that you must be at the scene in 2 minutes, and says nothing about having started a rescue. therefore to save time raf firemen wear their crash kit while on duty.