First, the term Spaniel is no longer used. They did away with Spaniel in the Brittany's name back in the 1980's. So basically now you have the American Brittany and the French Brittany. Just a little FYI.
Now about running your Brittany in cold temps. I've had Brittanys for 20 years and live in South Dakota. Cold temps have never seemed to effect my dogs and I've hunted them in some extremely cold weather (below zero). As long as the dog is moving it will be fine. You can use a vest to help but sometimes in snow the vests will fill with snow. When I get back to the vehicle I try to remove any snowballs that have accumulated on the dog before I kennel them. I've always hunted out of a SUV so my dogs have a warm environment when kenneled. Not sure in very cold weather it would be a good idea to put any wet dog in the back of a pickup with no heat. I wouldn't worry about the temps. Chances are the dog can withstand more than you can.