Adding sour cream isn't going to make it taste weird, so you could use it no problem, in fact it's a really good ingredient - more on that later. You can use double cream, but you've got to make adjustments - it's not a straight substitute for 2 reasons:
- Fat content: sour cream is 20% fat, double cream is 50%. If you substitute double cream you'll be increasing the fat in the recipe by about 50% (the butter is 80% fat). If you mix 1 part double cream and 2 parts milk and add that you'll have about the same amount of fat. However, sour cream will be much thicker, so you will get a runnier batter, and you will not get the same structure in the end result
- Acidity: Sour cream has lactic acid added to make it tangy. Your recipe. Many recipes have baking soda in it, and may have baking powder as well. Baking soda must react with an acid in order to work, if you substitute double cream for sour cream you are removing the acid, so you'll need to replace the baking soda with baking powder. (Note, I first wrote this when there was no recipe in the question, I'm keeping it because many recipes do use baking soda, and I like a complete answer as it helps a broader set of readers)
When the acid in sour cream reacts with baking powder the acidity is reacted away, if there is baking soda your batter won't be sour anymore. In the case of your recipe there there is only baking powder, so any tartness is the intent of the recipe. Perhaps it is there to help balance out the bitterness of the coffee.
So, you can substitute double cream with some measure of success, but I personally would trust the recipe and use sour cream.
