Hmm, Residency
After almost 20 years of being in the RAF (having joined from a Commonwealth country) I still occasionally encounter 'nationality' issues, in spite of having a British passport. It's generally to do with access to particular information, again irrespective of having the appropriate clearances and the requirement to access that information, and the immutable fact I was born 'Abroad'.
However, Captain King Damian, follow Sand4Gold's advice and don't take no for an answer from the Careers Office. Get yourself over here and show grit and determination - as well as an oustanding aptitude to do the job. There are other routes (such as joining the British army for a few years) that would soak up this requirement - just make sure that you have plenty of head-room to apply before you get too old.
Good luck
WP