Googling tells me that in Australia, a beef animal would be expected to drink 45-60L per day. I guess an outwintering hardy beast in Wales would drink less

. But then, if she’s lactating, more. A different search said a lactating cow is likely to drink 3-4 x the quantity of milk she produces. More if she’s on hay and or it’s hot and dry.
Our Jersey probably peaks at 15-20L milk a day (we don’t push her), so in summer when it’s hot and dry probably drinks 50-100L (11-20 gallons) of water a day.
I’m guessing each Highland cow will produce up to a couple of gallons of milk a day? So perhaps each would drink 6-10 gallons of water a day in a hot summer.