In this context, 'all this' refers to the various types of food and drink the writer has consumed. The phrase implies that the writer has had a meal or snacks, and to finish it off or accompany it, they drink Coca Cola. It emphasizes the casual, perhaps even indulgent, nature of the meal. The use of "wash down" suggests that Coca Cola is being used to cleanse the palate or as a refreshing beverage to complement the food.