Lets talk about watering.
Living in England means most of the year rainfall is sufficient to keep the grass roots supplied with moisture. However we do get the occasional very dry spell where watering is required. It is the top 100mm (4 inches) of soil that is important to keep moist and one major drawback of letting your lawn dry out too much is that some weeds are more drought resistant than grass so they can get a good foothold while the grass around them is struggling.
Flooding the lawn with water regularly however is not ideal as the soil water-logging the soil will stop air getting to the roots so it needs to dry out in between watering which will help promote deep root growth. Conversely watering little and often will help the spread of moss and other undesirables and will cause the root system to stay shallow.
So the main points to remember are when watering get enough water on the lawn to penetrate to 100mm, but only water once or twice a week, and do it before the lawn starts to suffer and this will yield the best results, always remembering that every single lawn is different and you need to match your tactics to the conditions you have.