, I've just been to the supermarket, and while I was in the dairy section picking up a couple of containers of double cream, I noticed some shallow tubs of creme fraiche nearby, so I bought one of those too out of curiosity.
Here are the respective manufacturers' ratings of the two brands of double cream that I've had, together with the ratings of two brands of creme fraiche (one of which I've just bought, and the other of which is from the same maker as one of the double creams). They're not standardised on the same serving size, because two of the manufacturers specify the serving size by volume, and the other specifies it by weight:
Double cream, brand A: 16.5g fat, 0.6g carbohydrate, 0.5g protein
Double cream, brand B: 15.5g fat, 1.3g carbohydrate, 0.6g protein
Creme fraiche, brand B: 41.8g fat, 2.3g carbohydrate, 3.2g protein
Creme fraiche, brand C: 18.5g fat, 1.5g carbohydrate, 1.1g protein
However, the thing that matters is the ratio of wanted non-insulinogenic content (fat) to unwanted insulinogenic content (carbs and protein). Here's what I get if we standardise the three specifications against 100g of fat:
Double cream, brand A: 3.64g carbohydrate, 3.03g protein
Double cream, brand B: 8.39g carbohydrate, 3.87g protein
Creme fraiche, brand B: 5.50g carbohydrate, 7.66g protein
Creme fraiche, brand C: 8.11g carbohydrate, 5.95g protein
So, while brand A double cream will obviously be my preference, there isn't really all that much practical difference between the brand B double cream (from a somewhat boutique manufacturer) and the two brands of creme fraiche. One creme fraiche has a little less carbohydrate and a little more protein than brand B double cream, and the other creme fraiche has significantly less carbohydrate and significantly more protein. It's a matter of swings and roundabouts.
The creme fraiche seems to me to be a little less solid than double cream, and double cream doesn't have the sour note of creme fraiche.
So, if your creme fraiche has a fat-to-carbs/protein ratio similar to ours, then you're not missing out on much if you can't get your hands on some double cream -- plus, I gather that you're very partial to creme fraiche.
I don't know how the sour note of creme fraiche would go in platinum coffee, though.