Which product for white chapatis?

Just look for a "white" or "fine white" - many of the chapati flour companies make half a dozen types. Don't expect to find the full range at any of the "middle class" emporiums like Ocado, Sainsbury, etc, you'll have to dig a little deeper than that; but it depends where you live. My local Asda does 2 or 3 varieties, Sainsbury's... none.

Personally, I don't like Chakki flour at all as it makes doughy soft chapatis & I like mine with a bit of dry bite to them.
Basically, the finer & whiter the flour, the more doughy the chapatis, so going for whiter than white flour isn't always a winner, unless it's for health reasons as opposed to flavour. A 'medium' flour is going to make chapatis like your local takeaway makes.

This is a fairly random link rather than an actual recommendation, but just to show the varieties available - Elephant Atta's flour range
btw, I have nothing against East End's food range, I use it a lot - Elephant's just looked easier to navigate.

As for additives - I think you're just going to have to read the back of the pack. I think these days, 'fortification' is pretty much the norm for white flours; wholemeal is less likely to have additives.