The Iraq War had nothing to do with Democracy. There were other countries just as undemocratic as Saddam's Iraq, namely Saudi Arabia, UAE, Kuwait and the rest.
Saddam's mistake was to plan the assassination of George Bush.
http://www.fas.org/irp/agency/doj/oig/fbilab1/ 05bush2.htm
They could let him off for genocide in the war with Iran, of gassing his own people, even invading Kuwait did not see him removed, but plotting to kill the American President is unforgivable and his
card (literally) was marked.
Revenge was the motive, the oil was supposed to pay for it, and Britain's token involvement was to prove it was not solely an American operation.