Just read this page, it sums up the argument quite nicely..
http://www.dft.gov.uk/pgr/roadsafety/research/rsrr/theme1/bicyclehelmetsreviewofeffect4726?page=13#a1058
Section 9: Conclusions
What relevance does the evidence reviewed have for bicycle helmet promotion in Britain?
Unwin (1996), when considering the context of the British legislative system, has put forward four criteria which must be met before bicycle helmet wearing is enforced. These criteria are:
(1) There must be a high level of scientific evidence that bicycle helmets are effective in reducing the rate of head injury to bicyclists.
(2) The benefits to society and others of mandatory bicycle helmets must be convincingly demonstrated, mandatory bicycle helmets cannot be justified simply to protect individual adult bicyclists.
(3) There must be widespread agreement, ideally by a large majority, that the potential benefits of compulsory bicycle helmets outweigh the infringement of personal liberty and other disbenefits.
(4) There must be good evidence to suggest that compulsory helmet wearing would not make the public health benefits of increased levels of bicycling significantly harder to obtain.