If a written constitution had not protected a number of other countries from tyrannical governments, then why would it protect Britain, perhaps the lack of a written constitution provides a better a more organic approach to the evolution of checks and balances between the three arms of government. In Britain's electoral system elections must be called at least every five years, this makes long term planning difficult as each change of government can cause a change of direction. The current system of elections and constituencies can often lead to what is effectively a one party dictatorship. For example the second term of the Thatcher government and the new labor government of 1997, they elect party like this with an enormous majority so that they can implement. Their policies, this is the opposite to the what happened in America when Clinton was elected because he had such a small majority he was unable to actually make any of his policy's executed. The key to these one party dictatorships is that the electorate has the ability to change governments if they are unhappy with how the previous government has operated. Even the Government can find that if it passes a law the judiciary still has the power to overturn it if they believe it is not in line with the unwritten constitution.
Perhaps Britain is at risk from a tyrannical leader, but no more so than any other nation. Although there is not a written constitution there are conventions that are although not written imbedded into the system of government, for example the Queen must sign every bill before it becomes law. A cynical view of politicians never attempting to implement a totalitarian system is that because of the fixed life of a government it would be voted out at the next election and as the aim of all political parties is to stay in office this would be unthinkable - political suicide. Another major reason why the British political system is so safe is because the government would not be able to get away with any covert policies is because of the media who expose it and turn the public against that government.