Hi Lisa-Jayne, I am a displaced Brit now living in Canada and I can assure you your Berner is doing fine with the British winter. My Berners love the winter here and believe me it can be very cold, fortunately (so far) this winter the coldest we've had is -18C and still my dogs want to stay out and play. -20C and lower and they just take pee breaks and knock on the door to come back in:-) I have a large wooden deck which they like to lie on and that keeps them off the frozen or muddy ground. You might like to make a wooden platform with a shelter from the rain for your dog and that will keep the rising damp from making him/her stiff.
Rose Tierney
