If you can read French, there is some new evidence on the safety benefits of winter tires.
http://www.mtq.gouv.qc.ca/portal/pag..._avril2011.pdf
As with some other cold jurisdictions (e.g., Sweden, Finland), winter tires are required in Quebec from December 15 to March 15.
Quebec compared crash injuries since their 2008 mandatory winter tire law to the 5-year period before the law. They found a 5% reduction in injuries that can be attributed to winter tire use. This translates in an average of 574 fewer injuries per winter.
Of course this is cold Quebec and not mild Britain. The need for winter tires will ultimately depend on where you drive in winter.