Sugar – Liquid glucose in marshmallows

As far as I understand the basic chemistry, golden syrup should do well for you.

A different option is to dissolve a lot of sugar in some hot water and then to slowly reduce it to the right consistency, which should be more or less identical to corn syrup.

In Britain golden syrup is used in a lot of things, Lyle's brand is the one people know, though there are a few other minor brands. They tend to vary in thickness and strength of taste, but usually have fairly similar tastes. It is partially inverted sugar syrup, which has a fairly complicated chemical process to manufacture from byproducts of the refining industry. I doubt it's very easy at all to make at home.

Buy a whole tin, it is quite versatile, excellent in cakes and best of all on porridge.

Quick tip: If you need to add a large amount to a recipe which asks you to heat it, which is quite common, it is far easier to place the tin (with the lid off!) in the oven for a few minutes and carefully wearing gloves pour from the tin, as the heat makes it thinner and thus pourable. Same thing works for black treacle.