At what altitude are these colossal snow depths? I'm guessing at least 1000m, driving through North Wales yesterday there was nothing at all lying at 800m there. Since the temperatures this month have shown such little variation, and the rain has been so constant, it's entirely believable that there has been a "sweet spot" elevation wise where the temperature has been just right for loads of heavy, wet, accumulating snow for weeks on end. As the low-level maxima haven't been very mild- almost entirely between 5-10C- that level has probably been about 1000-1200m. By contrast in winters like 06/7 and 89/90 the maxima here were 12-13C for long stretches, which would push the freezing level above the highest British mountains. So even if it was very wet the snow wouldn't be able to accumulate so much. And even a freezing cold winter would be no useful if it was too dry.