To make the chips: Peel potatoes. Square each end and sides. Cut each into 3/4-inch slices. Stack slices. Cut into 3/4-inch sticks. Place in a bowl of cold water to soak for 30 minutes. Soaking removes starch, so potatoes will be crisp when deep fried. Heat vegetable oil to 350 degrees in a deep-fat fryer.. Drain potatoes, pat dry using paper towels. Remember to dry thoroughly, any excess water will splatter when they are added to the oil.
Dip frying basket in oil (this prevents potatoes from sticking to it), add potatoes. Lower basket into oil. Deep fry till just tender when pierced, and are starting to brown, 5-7 minutes. Lift basket out of oil, let potatoes drain. They are now partially cooked. After fish has been cooked, place the potatoes back in fryer basket, fry till hot, golden brown, 1-2 minutes. Drain on paper towels.
For the fish: Sift baking powder with flour, place in a large bowl. Make a well in the middle of the flour, add oil and beer, stir with a wooden spoon. Gradually add milk until incorporated. Do not overmix. Batter will become elastic. Let batter stand for 30-35 minutes, till thickened.
When ready to batter fish, in a copper bowl, whisk egg whites till stiff peaks form. Gently fold stiff whites into batter, using wooden spoon, until combined.
Using a two pronged fork, dip a fillet in batter, turning to coat thoroughly. Hold it over the bowl, so excess batter can drip off. Lower fillet into hot oil, turning once, fry till golden and crisp, 6-8 minutes. Remove to baking sheet lined with paper towel, and keep warm in the oven as you fry the rest. To serve, sprinkle chips with salt, decorate fish with lemon wedges. Serve with tartar sauce malt vinegar.