But it’s not like all the water just disappears after it enters the data centres.

It doesn't disappear as such, but it won't be in usable form.

The reason they want to make use of the water isn't as a heat transfer fluid to something else (well, they might use some water for that too, but that's not what's driving the consumption). They're evaporating it. The phase change from liquid water to water vapor consumes energy.

Evaporative coolers work on this principle.

So now you've got a bunch of water vapor blowing away in the wind, which you're not going to be drinking. It's not gone, as it'll turn into rain or some other form of precipitation somewhere else, but that'll be somewhere else.

Same thing some thermal power plants do

you probably have seen images of those nuclear power plants with cooling towers, and other types of thermal power plants will do the same, coal, oil, gas.

That being said, they don't really need freshwater, as long as they can set up some sort of evaporation system that uses seawater for cooling, doesn't clog up from salt or other stuff building up. More of a hassle to deal with than freshwater, but it still evaporates. The UK being an archipelago, seawater is not in terribly short supply.