If you are an A student then the UK Armed Forces won't take you until you have completed your education to its highest level.....Unless you insist. If you pitch up with 10 GCSE's (Or equivalent) they will turn you away to sit your A Levels (or equivalent) and likewise...if you pitch up with 3 good A levels they will tell you to come back after Uni.
Talk reaction is correct.... now is not a good time to join.... go and get a degree and join then.
I think the residency issue is only if you are not British.