We can not get BORAX in the UK, but the substitute here apparently is 'sodium Sasquicarbonate

"Sesquicarbonate, most commonly known as
sodium sesquicarbonate, is a natural, versatile mineral compound (a double salt of sodium carbonate and bicarbonate) used as an eco-friendly cleaning booster, water softener, and pH buffer, often replacing borax in laundry, bathing, and general household cleaning for its effective, yet gentle, alkalinity. It helps detergents work better by softening hard water, removes stains, prevents limescale, and is used in bath salts and even animal feed. "