I'm in the middle of class (school), and I don't have a ton of time to type anything out, so I'll point you in this direction:
http://www.thekitchn.com/thekitchn/food-science/leaveners-fats-the-science-of-great-biscuits-109416
Should help, covers the science. The answer will be found, plus more!
You need to make sure you use the right measurements when translating recipes from the US to the UK as the UK uses imperial measurements which are different from US measurements. There are also differences in cream fat content and egg sizes. Teaspoons and tablespoons are the same, so don't worry about them.
First, pint measurements, as the UK doesn't use cup measurements. The UK pint is 20% bigger than the US pint, so if you are using UK pints to measure you may be getting ratios wrong. Use Milliliters instead when translating. 1 US Cup is 237ml (I round up to 240). If you use UK pints as a measure instead of US you won't have enough baking powder in the ratio. In fact, I find UK baking powder and bicarb a bit weaker than US powders, so I increase those measurements a bit anyway.
What works for me is the first time I make a US recipe in the UK I weigh the dry ingredients and use weights every time I make the recipe after that, I can fine-tune it that way.
US light cream is between 10-30% butter fat, and heavy cream is about 36-40%. UK single cream is between 10-30% fat, whipping cream is about 36%, and double cream is 50% fat. So if you are right that US heavy cream is UK whipping cream, but mixing 2 parts UK double and 1 part single works just as well as many places don't stock whipping cream.
I doubt that your results come from the cream you are using though, the fat contents aren't that different. One thing that could be different is the flour you use. US and UK flours aren't completely the same. For my biscuits in the UK I buy 00 or purpose milled pastry flour, which is finer and better for pastry than the bog standard stuff. If you can't find it pick the flour with the lowest protein content you can find.
Hope this helps, let the forum know your results if you can.
Best Answer
Baking times are never exact, as there can be considerable variance in the product and environment. The three most likely variables to affect total necessary baking time are:
For this reason, there is normally a test or indicator for doneness. The best possible test is internal temperature of the biscuit, but this recipe does not give that information, and it is difficult to get an accurate reading for something as thin as a biscuit.
The recipe indicates browning as the test, but that does not indicate the state of the interior of the biscuit, only the exterior.
Still, there are several things you can do:
Assuming your oven temperature is accurate, the most likely factor is how thick or thin the biscuits are. The cooking time will pretty much directly related to this, all other factors being equal. Even the amount of cream has less effect than you would think, because doneness is related to the internal temperature achieved (the starches will hydrolyze and taste cooked around 195°F / 90.5°C) regardless of moisture level.
I have never tried the toothpick test on a biscuit, but fundamentally, they are quick breads, much like a muffin, so it may be worth trying: stick a wooden toothpick into the biscuit to its center. When it is pulled out, there should be only minimal slightly moist crumbs sticking to the toothpick when it is done. Wet crumbs indicate under baking, perfectly clean indicates over baking.
In the end, biscuit making is a matter of experience, and you will get better at it with more batches.