Look at the joint website of the Department of Health and the DfES, where you will find ideas for each key stage.
www.wiredforhealth.gov.ukteachers "Letter from Aunt Sally" for KS2 helps in teaching what's in a cigarette and the diseases smoking can cause.
"Smoking" is a topic in the "Lifebytes" section for KS3 and "Mind, Body and Soul" for KS4 deals with the effects of smoking on the body, as well as how to quit. There is also a quiz which delivers some alarming statistics.
The Guardian Unlimited website (www.education.guardian.co.uknetclasssmoking) lists a number of useful resources, including those produced by Action on Smoking and Health (www.ash.org.uk), the BBC (www.bbc.co.ukhealthkidssmoking.shtml) and the Roy Castle Lung Cancer Foundation (www.roycastle.org).