In reply to John_Hat:
Best way to keep warm? Stay active.
> I get very cold very quickly, and then take ages to warm up
Don't let yourself get cold. Or, if you do, put on your warm layer and get active.
A warm layer won't generate heat; it will only help conserve what you have. And you'll only generate decent amounts of heat by being active.
So, as soon as you stop being active, put on the warm layer to conserve your body heat.
Sorry if this is sucking eggs...