That's averages over the whole countries though - the north and west of the UK (particularly the Scottish Highlands and the Lake District) are a lot wetter than the south-east where London is.

Also, "volume of rain" might be misleading because it doesn't give you a feeling for how often it rains. The Highlands in particular can have a light rain that falls for weeks at a time (or at least seems like it if you are in a tent).