Water is precious. In fact it's becoming even more so, with rivers experiencing significantly less flows due to climate change. Reservoirs too are struggling to maintain supply as rainfall and the weather becomes more unpredictable. Plus, it costs money to purify water.
So saving water makes sense on every count. Toilets typically account for 30-40% of total household water use. Older toilets can use as much as 9 litres of water per flush. Even modern ones can average around 6 litres. Try using Save-a-flush sachets or Hippo Water Savers in your cistern - they can reduce the amount you flush by one litre without losing efficiency.
Water saving devices (e.g. Variflush) are also available free from many water companies. And if you're considering replacing your toilet, look for one that uses less than 5 litres to flush.
Various water saving taps and water-efficient taps are now on the market. Or you can fit a device like Tap Magic on your existing tap - it can reduce wastage by up to 70% while still giving you the normal full flow.
A water butt is a great way of collecting rainwater to use on the garden and can save many gallons of water.
It's possible to collect rainwater in underground tanks for use in flushing toilets and general cleaning. Construction Resources have information on this.