"there is a 50% possibility that this winter will be a cold one!" ... he, he! Very funny. Don't you see the humour in it? If it was American, I would wonder, but I find in that saying the typical British humour we've come tot love, on this side of the "little pond" (Norway, across the North Sea).
Anyway, as you know, these kind of predictions are the result of complex modeling of things like the North Atlantic Oscillation and perhaps even, El Niño. In other words; trying to find a pattern in chaos. I don't think those are done by the Met Office but rather, by the World Meteorological Organization that gathers are redistribute data to all member countries.
Anyone could make a guess like that but they go into the forecast models and look at a wide range of them. All forecast models show a different pattern and without the ability to understand what you are looking at and have the physics and math background involved in a meteorology degree, there's a limitation between a guess anyone could make and an understanding of what all the models show.