I suppose the first question is - What do you intend to do with the water?
If you just want to use it to water the garden, top up the pond or wash the car, then a simple barrel or butt will do the job, but if you want to use it for more domestic purposes the system starts getting more complicated.
More than likely the water metering will allow a set amount of free treated drinking water for every person within the household, so there’s no need to design a rainwater harvesting system that will treat the rainwater to drinking quality. It would be better to use the filtered rainwater for such things as washing clothes and flushing toilets, and as far as I know, that accounts for about half the domestic water use.
So how does it work? Rainwater is gathered from the roof and as it is going down the ‘downpipe’ it passes through a plastic yoke that filters out the leaves, and beetles and most of the bird crap, and then into either a ‘butt’ or an underground tank. From there the water is pumped up to a header tank in the loft, and piped down to the toilets on washing machines. It is important that this water supply is kept separate from the rest of the plumbing for obvious reasons.
The cleverer systems use a solar powered pump to replenish the header tank during the day so that there is no need for mains electricity.
I think it’s worth doing, and if anyone out there is serious about installing a DIY system I will assist from my keyboard in any way I can (within reason).