How to make double cream 48% for tart filling (not for whipping)?
If something like Agar Agar, or a small amount of Carageen is melted into the warmed butter (powdered form, while whisking/lightly whipping after coming off the heat, or bloomed into the warmed heavy cream, it would add the extra thickening that you are talking about without actually changing the flavor, altering the "double cream" (if being used as a filling for say, a tart), set up lovely and invalidates your argument.

I got my Masters in Pastry with a specialty in Alternative Baked goods.  Molecular Gastronomy is a significant part of that.  I would absolutely use powdered product to start, such as the two I mentioned...commonly available in Whole Foods (in the United States), some shops that specialize in Vegan products (as both are Vegan), though the amounts required aren't an exact science (as it depends on moisture in the air where you are, the fact of are you using European Butter (higher fat content and less water) than American Butter, if the butter is salted or not, how hot the melted butter is when you add the gelatin of choice, altitude and the speed at which you do the process.

For the amount of double cream mentioned above, I would approximate roughly 1/4 teaspoon of Agar Agar, OR 1/3 teaspoon of Carageen.  It will make the process a little more labor intense, however, the results may very well surprise you.

Just my two cents...Do with it as you will.  I hope you enjoy, and if it works, please, do a follow up post.