The origins of the term Tommy Atkins as a nickname for the British (or rather English) soldier are still nebulous and indeed disputed. A widely held theory is that the Duke of Wellington himself chose the name in 1843. Lt. General Sir William MacArthur, however, in an article in the Army Medical Services Magazine, says that the War Office chose the name Tommy Atkins as a representative name in 1815. Specimen forms of the Soldier's Book issued for both the cavalry and infantry in that year bore against the space for the soldier's signature "Tommy Atkins, his X mark". With the improvement of education "his X mark" was dropped.

The nickname, however, was used before 1815. In 1743 a letter sent from Jamaica referring to a mutiny among hired soldiery there said "except for those from N. America (mostly Irish Papists) ye Marines and Tommy Atkins behaved splendidly". At about the same time the English soldier was also nicknamed "Thomas Lobster", because of his red uniform coat.

The poems of Rudyard Kipling helped to popularise the name throughout the last quarter of the Nineteenth Century and especially during the Boer War (1899-1902). Thus, by the early Twentieth Century "Tommy Atkins" or "Tommy" was the almost universal nickname for an English soldier. During the First World War (1914-1918) the troops rather despised the name and only used the term derisively or when imitating the style of a jingoistic newspaper like John Bull.

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Sources:
How the regiments got their nicknames by Tim Carew (pub. Leo Cooper, London, 1974) ISBN 0-85052-054-1
Journal for the Society for Army Historical Research  Vol 1, No. 135; Vol 1, No. 18; Vol 2, No. 9-10; Vol 9, No. 175