250-300 I believe.
According to this link:
http://www.september11victims.com/september11victims/COUNTRY_CITIZENSHIP.htm
67 were British citizens or about 2.3% of all those killed that day on US soil.
70 British soldiers have died in Iraq so far (based on the latest information I could find) and therefore they have taken about 6.5% of the losses fighting the war so far.