Water bills are high. The rainwater harvesting is off the shelf stuff. Mandatory in the BenNeLux countries in new builds. It is worth it with a 7 year, or less, payback. If you want really low bills install a rainwater harvesting and waste water reclaim system - a tank in or under the garden. Your water charges drops as well as less water goes down the sewer. It is one of the few retrofit measures that have a quickish payback. The other is the GasSaver used on some condensing boilers - 3 to 5 years.
You can justify the costs of rainwater collection quite easily as the whole kit is cheap using a lot of used orange juice containers. You could use a basic septic tank (around £500) and a submersible pump (around £50). Since you are only using collected rainwater for lavatory flushing there's no requirement to filter it, but if you did want to use it for washing etc. then you should be able to find a fairly simple filter system for under £100. Installation adds on a bit. The downpipes from the roof have to feed into the tank which overflows into the sewer.
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