Buy powdered glucose from Holland&Barratt (or other 'health food' shop) rather than dissolved to a syrup from Sainsbury's (or other supermarket, grocer, or cake-bakers' supplies store).
Powdered is about 4x cheaper. (Or rather, you get maybe 4x more Glucose for a few pence more.)
Once dissolved in water, its exactly the same stuff.
Don't skimp on the Glucose, maybe even use a little more than Rosti's video suggests.
While at H&B, buy some Fructose as well.
Its a great 'humectant' - keeps stuff moist. So transfer it to a sealed jamjar immediately after opening, or it'll transform itself into syrup.
Adding a little Fructose (maybe half the Glucose quantity) to your fondant helps prevent it drying out.
The old-school 'inversion' with (acidic) Cream of Tartar is not advised for bees nowadays (although I don't think actual harm to bees from this specific process has been proven, its entirely possible) because it could (rather than always does) produce (bee-) harmful levels of HMF. Its rather like the "bacon can cause cancer" concern in human nutrition. You can make nitrosamines in fried bacon, and nitrosamines will cause cancer, but the number of cancers seen is nothing like the number predicted from the amount of bacon consumed.
People have, and some still do, use acid inversion - without wiping out their bees. But its not "best practice."
You don't need to do acid inversion for your bees.
'Inversion' is about creating some Glucose and Fructose from the ordinary sugar.
Simple thing to do is just add Glucose and Fructose rather than inverting your own.
Incidentally, honey, (and "Invert Syrup" like Ambrosia), contains Glucose and Fructose; in honey they are in varying amounts depending on what the bees have been foraging on - which is one reason different honeys 'set' differently.
Rather than making it, some people find it even simpler to buy "Bakers Fondant" and not worry overmuch as to how it might have been made.
I use a calibrated kitchen thermometer (a Thermapen), and IMHO the temperature suggested in Rosti's video for the final boiling temperature might be a degree or so too high.
Certainly stop slightly too early rather than too late. (Slightly too late, too hot, gets you a hard rather than paste-y result.)
But do note that the main use of fondant (US candy) is as a feed that can be given during the winter, after it has been adjudged that stores are running down too fast.
