There’s a practical reason for this. In a lot of UK homes, mostly those built from the 1940s to 1960s, cold water comes directly from the mains supply and hot water comes from a local storage cistern, often situated in the attic. They’re kept separate to avoid cold “drinking” water being contaminated by hot “washing” water which isn’t fresh. My older home doesn’t use this system and I have mixer taps in both my kitchen and bathroom.