I'm not too worried about experiencing low flying temperatures really in Guernsey, though the wind chill would be something to take due care of. I'd really like to fly some times in the Alps/ mountainous areas so that would be where my main concern would be!
The pocket method is something I have considered, but there are still plenty of stories of people having failures at 20% or even higher. I never push my battery past 20% even in normal flight conditions, it's bad for the battery degradation. Have to keep it between 20-80% for Li based batteries.