Find the Average Rainfall UK. What is the average annual rainfall in the UK? How many days a year on average does it rain in the UK? Where is the most rain in the UK? These are some of the questions this page will try to answer. How much does it rain in the UK? UK has a moderate sea climate which allows rainfall throughout the year. Average annual rainfall in the UK is around 1154 mm. It rains the most in October (127.1 mm), and the least in May (70 mm) on average. How many days a year on average does it rain in the UK? A research which…