The eight-glasses rule is a slogan, not a study. Fluid needs vary with body size, heat, exercise and how much soup, tea, fruit and vegetables you already eat. Many people are fine on less than the poster. Some athletes and older adults, whose thirst signal dulls, need to be more deliberate.
A practical check is dull and effective: pale straw-coloured urine, a mouth that is not dry, and a headache that is not your only personality trait in warm weather. Coffee counts. Tea counts. Water-rich food counts. Alcohol does not cancel the rest, but it is a poor hydrator.
Overdoing water is rare and dangerous, mostly in endurance events or in people who force litres on a dare. The greater British issue is not dehydration so much as what comes with the fluid: sugary drinks, and now a fashion for expensive bottles that imply tap water is a moral failing.
Drink when you are thirsty. Keep water to hand if you forget. Do not outsource common sense to an app that congratulates you for swallowing.