Bassenthwaite Lake catchment area

I have blogged before about water catchment areas having estimated them from maps but this time I found the catchment area value given. Suppose an inch of rain falls over the catchment area - that might be a decent day in Cumbria. That's 0.025m. 350 square km is 350 million square metres so the volume of water falling in the catchment area would be of the order of 10 million cubic metres or 10 billion litres. I found a figure that on average a person in the UK uses 150 litres per day. That would mean the volume of an inch of water in this catchment would sort out the whole UK for a day. So why are we short of water?