The Huffington Post has a piece comparing open atheists in government in the USA with the UK.
They note that only two American legislators have only ever really professed non-belief: Pete Stark and Barney Frank (the latter admitting it after leaving politics). Meanwhile, the current deputy Prime Minister of the UK, Nick Clegg, leader of the Liberal Democrats, is an atheist, as is Ed Miliband, Labour Party and Official Opposition Leader. Conservative Prime Minister David Cameron is a Christian but has brought in gay marriage and UK politicians are routinely reminded that Brits “don’t do god.”
Further to that, the British Humanist Association maintains a Humanist Caucus with over 100 elected MPs and unelected Lords in the three major parties.
The closest Canada has had to an atheist Prime Minister might be Kim Campbell, who is listed as a “lapsed Anglican,” although many Liberal Prime Ministers may not have been as Catholic as they professed. Pierre Elliot Trudeau was reportedly a board member of the Humanist Fellowship of Montreal. Several past federal party leaders have been reported as atheists, including Stephane Dion and Gilles Duceppe. Few Canadians wear their religiosity (or lack thereof) on their sleeves though.