Relief or Orographic rain
As well as frontal rain, the UK is also affected by relief or orographic rain.
Relief or orographic rain is formed when air is forced to cool when it rises over relief features in the landscape such as hills or mountains. As it rises it cools, condenses and forms rain. The highest rainfall totals of over 1,600 mm per year occur in the mountain areas along the west coast. There is often a rain shadow effect whereby the leeward (downwind) slope receives a relatively small amount of rain.
The diagram below shows how relief rain operates.