It's not just a lack of rain but a lack of rain in the correct places. The greatest rainfall is in the south and west, due in part to geography as well as most common wind direction. However, the greatest population density is in the south east. Water in the south east comes mainly from natural underground reservoirs where water gets stored in rocks such as chalk. These are heavily dependant on wet winters to be topped up before the summer.
Accept it or move to Bristol, it never stops raining here!
P.