At a guess I would say it's the ABS working the way it is intended to. What happens when you press the brakes on a slippery surface is that individual wheel sensors detect if individual wheels are sliding and release the brake on that wheel so braking can be accomplished by the wheels that are on a solid surface. The catch is that if all four wheels are on slippery surfaces then the brakes simply will not brake the car. But they would not anyway. Even if you had been able to lock all the wheels the car would have slid onwards, all wheels locked. If the cold weather continues, more Brits will have to consider keeping a set of softer winter tyres that are less likely to slip in icy conditions.