The British Army was one branch of Great Britain's military. The Doctor has worked directly with the British Army on a number of occasions; he has also been involved with them via UNIT.
Following the formation of UNIT the British army appointed a UNIT liaison officer to whom the officer in charge of the British branch of UNIT report. At the time of the Auton incident Maj. General Scobie was the British Army's UNIT liaison officer. UNIT troops were usually members of the British Army serving on detached duty and as such were subject to the orders of British Army officers of superior rank or could issue orders to British Army soldiers. (DW: Spearhead from Space)
Whilst the Battle of Canary Wharf in 2007 raged, the British Army fought the invaders across London. (DW: Doomsday)