The main roads are always gritted, it is just the side roads, shared paths and the country lanes which tend to be icy. The country lanes being the most likely as it is usually(always) colder out in the countryside.Thankfully for my route to work I can't ever imagine needing to use studded tyres. I managed to commute into central London in snow a couple of years ago on 23mm (interesting experience!) so with 25mm marathons on I should be ok.
Even in the town the odd unexpected ice patch can be found, normally after someone has thoughtfully defrosted their windscreen with hot water from a kettle.