How long would Loch Rannoch last us?

Mrs B asked a wonderful question: for how many days would the water in Loch Rannoch could supply the UK? I had in mind to do a volume calculation as a triangular cross-section multiplied by the length of the lake. Google searches gave the depth as 134m and width as 1000m. Length is 15km. So volume = 0.5 x 0.134 x 1 x 15 = 1 cubic kilometre. I was pleased to see that was a volume someone else had come up with too. There are 10^12 litres in 1 cubic kilometre. The web reckons the average UK resident uses 150 litres per day. Wow! Population of the UK is about 65 million so we would use 9.75 billion litres a day. That means Loch Rannoch would last us about 100 days. It's a good job it rains a lot.