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).
Most of the Bay Area gets about the same amount of rain as London. The key difference is that in Cupertino/Los Gators/Palo Alto it rains less often but with more intensity.
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).