Mine is apparently good to -20 but I've only used it at -5 max
For ease of use, I often just fasten it with the buttons but that lets a lot of cold in. For it to work properly, I have to use the zip to the top with hood on. If used like that, I'd say it works well for UK weather. I'd have a max of two layers on underneath but more likely one layer.
I think with a proper long sleeve "base layer" and a wool top and the coat done up properly, it would be fine for much more than -5