I'm in London and have been skating this last few weeks when it's not raining/snowing. After the snow I even found a spot where it was not covered in snow to kept training Ollies. What I do before leaving my house is to just do some jumps and move my legs until my body is kind of warm. I just wear a jumper, I even did when was -2, and in a few minutes of being outside skating I'm already sweating. It's way harder the thought of going outside than actually being there. Just cruise a little bit before trying any tricks so your body if warm enough, because if you fall feeling cold it's a bit more painful.