Winter tires are made with softer compounds in order to have some flexibility in cold weather. All-season tires get hard when cold and the tread doesn't work too well with snow or ice.
Winter tyres provide about 10% more grip on cold roads, I prefer to drive 20% more slowly. In Germany if you have an accident in the cold and no winter tyres I believe your insurance is invalid. Using a temperature of +7C in UK is useless, you can have that temperature or below at almost any time of year.