Living in Finland, we have mandatory winter tires from december to february. Most people use studded tyres. Whether you need winter tyres depends on you: how many days of ice/snow you have in year, do you really have to drive every day 24/7/365 (366 this year)? Low temperature is not the most important thing, it is most slippery around zero Celsius.
Even more important than tyres is your attitude an carefulness: Drive slowly and keep a loooong distance between you and the car in front of you! If you cant avoid a crash, try to hit the ditch rather than a car, that usually means less damage.
http://edition.cnn.com/video/#/video/world/2011/02/07/qmb.fc.helsinki.snow.cnn

For the UK, the temperature is one of the most important factors tbh.

In Romania it is mandatory to have winter tires when the conditions impose them. It doesn't matter if it's April or June, it it snows, there's ice on the road or any sort of snowy thing thingie precipitations, you need to have them. If you don't and they catch you, you get a 500£ fine and bye bye registration certificate until you take another MOT with winter tires on. And you need to put the car on a platform to take it home or else you go to jail.

And of course, our minimum wage is around 120£ and winter tires would get you anywhere between 300 and 450£ so yeah, pretty freakin' expensive but something we need to live with in this ***t for country we call home.