The Royal Military Academy
The Royal Military Academy at Woolwich, which finished up at Sandhurst, was established in 1801, but potential officers were not obliged to attend it at first, and there was no guarantee that those who did would receive free commissions.
Artillery and engineer officers, meanwhile, could purchase neither first commissions nor subsequent promotion. All had to pass out from the Royal Military Academy and then advance by seniority.
'... until World War One it was difficult for an officer to survive without private means.'
The fact that most officers came from a relatively narrow social spectrum did not matter much in peacetime, but when the army expanded for World War One many surviving pre-war regulars received promotion beyond their normal expectation. Britain’s first citizen army was commanded, at its higher levels, by officers from the old army.