I'm considering some Schwalbe Marathon Winter tires so I can continue commuting to work during the UK winter ( -3°C to +8°C, with some snow and ice). However, I only have V-brakes. Are those brakes going to be acceptable in sub-zero temperature, occasionally while snowing?
Yes. They will be fine. Plenty of people use them in the Minnesota Winters without problems. Just make sure they are always in good shape, which is something you have to do with all brakes.
I've had some horrible experience with v-brakes in snow - it was like having no brakes at all.
I was lucky to find out the ineffectiveness of the brakes in a relatively safe situation, but that was like a revelation about the shortcomings of v-brakes. (Otherwise they've been quite reliable for me - even in the mountains.)
Keep in mind, the snow may come onto the rims unnoticed and breaks is a sort of things which tend to take you by surprise.
So, in my opinion - this is risky.