Tommy is a slur used to describe British in a derogatory way. Tommy Atkins was a fictional soldier used to represent all British soldiers for morale purposes during WWI.
http://www.britains-smallwars.com/Anecdotes/TommyAtkins.htm
- Origin in dispute.
The word Tommy is a racial slur that has been used historically to degrade and dehumanize british. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to british. If you are unsure about whether or not it is appropriate to use the word Tommy, it is best to err on the side of caution and avoid using it in any situation.