The bottom line as I see it, is that we import £billions of goods from Germany every year, which sustains thousands of German jobs. There is no way they would suddenly stop selling their BMWs, VWs et al when we leave the EU. As for renegotiating 'existing trade', both sides would ensure a quick resolution for the same reason.
As for the militant French farmers, how long do you think they would just stand back and let Brussels stop then selling their Champagne, wine and farm products to us?
Not disagreeing with you but it strikes me that if a lot of the people currently arguing about this issue hadn't brought german cars and filled their Italian fridges with French wine and cheese we wouldn't be having this discussion about trade.