The British Army recruited its commissioned officers from the wealthy governing classes. Having a son become an officer in the British Army or Navy brought added status and prestige to wealthy families. Before 1872, the son of any wealthy man could become an officer by having his father pay the military for his position. Most began as Captains or Lieutenants. Others started out as Ensigns in the infantry or Cornets in the cavalry up to 1870. In 1871, these both became Second Lieutenants. If someone had enough money, they could purchase a Field Marshall, General, Lieutenant General, or Major General position, but often the army filled these through promotions of regimental Colonels, Lieutenant-Colonels, or Majors. Commissioned officers also filled the positions of Adjutant, Quartermaster, and Paymaster. Doctors and ministers became army surgeons and chaplins. The Royal Navy and Marines employed the same system.
Enlisted men and noncommissioned officers–Sergeants and Corporals–came from the poorer classes, voluntarily between 1780 and 1914, by conscription before that. It was common for men to be rounded up by conscription gangs to be hauled off to enlist in the army or navy. Enlistment was usually for life except for limited enlistments permitted between the Napoleonic Wars, lasting from 1803 to 1815 and 1829. But in 1847, the British Parliament changed the length of enlistments for infantry recruits to 10 years. In some cases a soldier could buy his way out of the army.
While there were wars before the beginning of the 18th century, six major ones in which the British fought occurred from 1740 to 1900. If you know that your ancestor fought in the War of Austrian Succession (1740-1748), the Seven Years War (1756-1763), the American War for Independence (1775-1783), the Napoleanic Wars (1793-1815), the Crimean War (1854-1856), or the Boar Wars in South Africa (1899-1902), you can look into records from those wars.
To find the name of the regiment to which your ancestor belonged, you need to first check family and home records, as well as church and census records. These aren't the usual family records genealogists know so well, but the records of births, marriages, and burials of army personnel compiled by army chaplins from 1796 to 1880. You can order certificates for these records from The General Register Office (GRO) in London (www.gro.gov.uk) Regimental registers of soldier's marriages and the births and baptisms of their children from 1790 to 1924 are also at the GRO. The Public Record Office in London has copies of the Monthly Station Records which show where the army stationed individual regiments each year from 1759 to 1865.
Once you know the British regiment in which your ancestor served, you can search military pension records for soldiers discharged from 1760 to 1900. These records contain particulars of age, birthplace, and trade or occupation on enlistment, a record of service, and the reason for discharge to pension.
Most countries have varied ways of keeping and organizing records of their armed forces. The British are no different. They have kept records of each army regiment–often subdivided into cavalry and infantry–and naval vessel. Within these general records, you'll find lists of soldiers and sailors by rank, as well as major wars and date of discharge. Extensive histories of major military campaigns, including those awarded medals for their bravery. Besides army records, you may also consult those for the Royal Navy, Coast Guard, Royal Marines, militia, as well as those for chaplins and surgeons, and ordnance personnel–miners, sappers, artillery and engineers. And prior to 1898, the British army also kept records of its troops stationed in far-off corners of the Empire.