Toolstation, Ebay, and possibly other retailers, sell several Barratine products, and sometimes wrongly describes shed and fence stains or varnishes as wood preserver, and attaches health and safety warnings that do not apply to them.
You can look at the Barratine website, and the Hazchem warning on the can, to check if it is a preserver. It will generally be considerably more expensive. You can also see the percentage, to compare weaker products that might be cheaper and less effective.
The preservers contain Permethrin, which poisons insects and is a danger to health, and a fungicide whose name I forget. These are (intentionally) poisonous and hazardous. Products that are not hazardous are not preservers , even if the product name looks similar