The British Army is the principal land warfare force of the United Kingdom. At the start of 1939, the British Army was a small volunteer professional army. At the beginning of the Second World War, 3 September 1939, the British Army was small in comparison with those of its enemies, as it had been at the beginning of the First World War.
https://en.wikipedia.org/wiki/British_Army_during_the_Second_World_War