I am not really sure what is your status and in what capacity you are working in British Army. If you have Indefinite Leave to Remain(ILR) in UK and had it for at least 1 year then you should be able to apply for Naturalisation. You did mention that you have been living here legally but you can be a student and living here legally. you must have ILR for at least 1 year before you can apply. Unless they have some exception for serving in the British Army. You can also check with Personnel Department of teh British Army

You can also call Home Office and talk to them about your situation. There are other people who will reply to you so you can also wait for their e-mail but no harm in checking with Home Office. 

i came to the UK in Sept 1999 as a student. since then i have being leaving here legally.

on the 01/12/2003 i joined the british army and i still in the army. i want to know if i qualify for naturalisation since i have spent over 5 years here and bearing in mind about the new changes to the commonwealth soldiers. during the period of being here, i have being out of the country for 28 days. do i qualify for residency since i have being here for 7 years?