Best product to brighten a white Dress

And then, when one of these methods has brightened it, be sure to wash it only with white laundry, to make sure it stays bright white.

I’ve use Glo white sachets before.

Vanish or the Lidl equivalent work well. Chinese shops sell a laundry blueing powder which brightens whites.

Vanish Oxi Action Crystal White or similar.

I would try Ace laundry bleach, and dry outside in the sunshine.