Herbal, you can use the holland and barret stuff with no probs...
just DONT use Lyles treacle as an alternative, it is molasses but also has a shitload of sulphur in it which isnt good for your ladies.
Unsulphured molasses is what youre lookin for.
"Grandma's" is a great brand, and is relatively cheap online in the UK..But you'll def get some kind of molasses cheaper at H&B healthfood stores.
there are roughly 4.54 Litres in a uk gallon...
I personally use 1tsp per 2litres,building to 1 tsp per litre for the final wks of flowering, but thats me...And i flush regularly to stop the medium turning into toffee and to clean off the roots lil and allow them to continue to respire.
Also be aware, that molasses use can increase your likelihood of pests in the grow space, esp in summer when the insects are lookin for nice sugary food.
Go steady n good luck with the grow
Frosty