How can i substitute 1/2 cup corn flour for cornstarch?

Is the recipe from England roots by chance? I think we have been through this before that in England ( Europe?) corn flour is what is called cornstarch in the US, Not corn meal or masa. However you probably could at least test out the similarity in function of your corn flour by trying to thicken liquid with it by the same ratios/ method as that used for corn starch. I usually eyeball it so I can't provide any ration such as X tablespoons of cornstarch to 1 cup liquid.

Oh okay, I'm guessing the cornstarch is being used to make a slurry to seal the spring rolls, yes? If so, then you could use an egg wash instead.

I'm sorry you didn't get an answer to this yet. Did you already make a substitution? Were you trying to thicken something? In the U.S. at least, corn flour and cornstarch are different, so there might be a better substitution option depending on the dish.