The Met Office released the following satellite photos of Britain:
On the left is how Britain looked like in May. On the right is how Britain looked like in July, after a heatwave that has now gone on for over 6 weeks.
The only reason we don't have water shortgages is because the acquifers filled up in Spring, when we had a cold spell. How long we can hold out though is anyone's guess.
Because the UK is normally a rainy place, there arn't any reservoirs. Instead water comes from acquifers tapping into the water table, and from rivers. The acquifers usually fill in winter, thanks to abundant rain, and deplete slightly in summer (there is normally a lot of rain in summer too).
What is happening now is simply unprecedented, and will prompt the government to look into long term water management, so that we can cope if the climate worsens.