Refiner’s syrup (also called “golden syrup”) is made, as the name implies, at a sugar refinery, not at the sugar mill which is where molasses is produced. It’s a by product of the making of white sugar, the final “molasses” that’s produced when white sugar is centrifuged. It contains mainly sucrose and water when it’s first spun out, but is treated with acid (or sometimes the enzyme invertase) to create a proportion of invert sugar. For more on sugar refining, see my (now integrated) primer on sugar making.
Refiner’s syrup can make a fine alternative to either molasses or corn syrup depending on the application, though since it tastes every bit as sweet as table sugar you need to be careful about overloading your recipe with sweetness. Lyle’s can be hard to locate in stores in the US, but you can make its functional equivalent at home by following these instructions.