The grape juice from Asda isn't concentrate, it's from concentrate.
Grapes are pressed, a lot of them from Spain, and then most of the water is evaporated to reduce shipping costs. That concentrate comes over to us where a lot of it gets watered down again, often pasteurised and then packed in cardboard cartons.
The difference is that the stuff in the cardboard cartons is pressed from mostly dessert grapes, whereas the concentrate sold in little tins is pressed from mostly wine grapes.
Carton grape juice is a great substitute for water but won't pack as much oomph as the GJC or Wine Enhancer in the tins.
PS: Aldi's grape juices aren't from concentrate and they are 85p.