The United Kingdom is one of the countries in the world that operates using the 12-hour time system, which means that the total 24 hours of the day are counted in two different periods according to the acronym AM or PM, depending on whether the indicated time is located before or after noon respectively. You have all the information in: «Time change and time system in the UK«.
You have all the information in: "Time change and time system in the UK".